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  #1  
Old 07-18-2003, 07:50 PM
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Article posted on Precision Shooting Website about Ackley Improved cartridges


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Good to hear from you and especially about those fine old-time Ackley Improved cartridges. As you know, Ackley was far ahead of his time…and far ahead of all the commercial cartridge companies in improved design. He was our last great cartridge experimenter who could and did write about his work. Today we seem to have arrived at a point where we have experimenters and we have writers, but the two almost never meet! The great Ackley handbooks, Volume I & II, Handbook For Shooters & Reloaders, is in need of updating, but it would be a major undertaking and would require the input of some top experimenter and/or writer. If you inspect the loading data in those two books you will immediately notice the outdated, discontinued powders and the fact that he almost never gave barrel lengths along with velocities. Also, a number of the cartridges in Ackley’s books listed data from other sources such as directly from the designer who estimated most velocities, optimistically. But in spite of these minor disparages, Ackley’s books are still the experimenters bible, just loaded with technical information not found elsewhere.

Before getting into your questions concerning the Ackley cartridges with the best percentage improvement, smallest improvement and the ones left in the middle, perhaps you would have an interest in Ackley’s background…just to pass on to your friends in the hot stove league involved in the Ackley arguments during cold winter days. Parker O. Ackley was born in Granville, New York, graduated from Syracuse University in 1927 and nine years later started his first gunsmithing business in Roseburg, Oregon, 1936. He was with the Ordnance Department during World War II and then moved on to Trinidad, Colorado to open what was to become one of the largest custom gun shops in the country. He also taught at Trinidad State Junior College where his gunsmithing school became world famous. He moved on to the Salt Lake City, Utah area where he continued his shop and did a great deal of experimental gun work. Along the way he became widely known as a gun writer where he passed on his knowledge of the trade. He left us all in 1989 at the ripe old age of 86.

The first cartridges designed by Ackley in the so-called Improved shape simply straightened out the tapered case body, giving the original factory designs a more straight walled dimension and leaving the shoulder angle the same. While this proved to reduce back thrust on the bolt, it still showed some case stretching in the neck and shoulder area that resulted in continued case trimming. When he changed the shoulder to something like 30 degrees the case lengthening slowed and by the time his designs reached 40 degree shoulders, all case lengthening stopped, within reason. It became common to reload those cases 15 – 20 times without having to trim them. Thus, the benefits of the famous Ackley Improved cases became reduced back thrust and elimination of case trimming. Case extraction became easier and more positive and loading pressures could be increased safely, resulting in higher velocities. Another interesting feature of these Improved chambers is the fact that standard factory cartridges can still safely be fired in the rifle. There are some die makers that still offer Ackley dies with the milder shoulders, so when ordering loading dies it is prudent to specifically request the 40 degree shoulder model.

Now to get on with your questions. There are more than 20 Ackley Improved cartridges, plus dozens of Ackley Wildcats, but some have about gone into obsolescence because the parent cartridge is no longer being loaded by the factories. Thus, brass is difficult to find. Of those that are alive and well today, the percentage of velocity gain over factory loadings ranges from about 3% up to 17%. A couple of others exceed this gain, but the parent cartridges are no longer being chambered for by the rifle companies. When we compare the Ackley Improved cartridges to handloaded standard factory rounds, the velocity gain ranges from zero up to a little over 12%. From this you can see there are a few Ackley Improved cartridges that may not be worth the trouble and expense to chamber for. But on the other hand, there are several that will simply blow you away with their new velocities, especially when compared to some of the much larger factory belted magnums, the big boomers.

There are many sources of cartridge velocities available today. These include factory listings as well as the dozen or so reloading manuals. This means there are many different barrels being used to arrive at those velocities. And that is why we, as handloaders and experimenters, must utilize several loading manuals in order to arrive at some norm that can be our starting point. When searching for the proper velocities to pass on to you to answer your questions, I first took the factory listed data from several cartridge companies and used their best figures. Then I utilized Ackley’s book figures, plus data from various other sources where barrel length and chronographed velocities were shown. This meant that some information differs from that found in Ackley’s books due to using new powders, stating the barrel lengths and chronographing over electric chronographs instead of the old pendulum style used so often by Ackley. In all cases pressures could only be observed by common shooter’s methods, that is, by checking primers, primer pockets, case heads, extraction, case life, etc. Generally, pressure guns using the crusher-gage method and resulting in copper units of pressure (c.u.p.) are not available outside the ballistics labs. This is also basically true of the more modern electronic-transducer gages that record in pounds per square inch (p.s.i.). These methods of discovering chamber pressures result in disabling the rifle by drilling into the chamber or at least attaching wires to it. It should be remembered that pressures given in c.u.p. are somewhere around 15% lower than those found in p.s.i. recordings, and when using various books giving pressures be sure to notice when two different methods are being used.

The best velocity gain of all the Ackley cartridges compared to the standard factory cartridge comes with the .25-35 WCF with a 117 gr. bullet and a gain of about 25.6%. The second best is the .30-40 Krag and the 180 gr. bullet showing a velocity gain of 19.3%. Both are rimmed cases and neither one is being chambered for today. Therefore, we will start with the third best velocity gain of 17% as found with the little .250 Savage when converted to the Ackley configuration and loaded with the 100 gr. bullet. Our rifle companies have chambered for the .250 Savage from time to time, but it is rapidly becoming obsolete in spite of the many knowledgeable shooters who use it regularly. The factory .250 Savage load is 2820 fps, while the .250 Ackley attains close to 3300 fps. This little speedster can equal or exceed the factory velocity of the much larger .25-06, listed at 3220 fps. And it is being done with 15 – 20 grs. less powder which means a great deal less recoil for the same velocity and trajectory. This is downright amazing…. And all this is being done in a short action. This .250 Ackley cartridge is not shown in any modern reloading book that I know of. Some books do show another .25 caliber, the .257 Ackley, that lands farther down the line in eighth place for best Ackley percentage gainers. I have used the .250 Ackley for both varminting and big game hunting with outstanding results.
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Old 07-18-2003, 07:51 PM
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The second half

For the next best gain, number four in the chart provided, I am taking my writer’s prerogative to show a standard cartridge that Ackley did not work on, but is now known as the 6.5x55 BJ Ackley Improved, the initials being my own. In the early 1990s I thought the standard 6.5x55 Swedish cartridge would show nice gains if it were improved as an Ackley. Little did I know that Ackley never did it. But to make a long story short, I got busy and copied the Ackley design into the 6.5x55, with a great deal of input from Dave Manson of Manson Reamers (PS advertiser). With the 140 gr. bullet the velocity gain is 13.7%. Standard factory velocity is 2550 fps, while the 6.5x55 BJ Ackley gets 2900 fps. This is another Ackley cartridge that I have used extensively for hunting, and I have heard from several readers who use it in long range competitive shooting.

Fifth in the chart is a surprise – the .30-30 Ackley. It shows a gain of 12.9% over the factory load with the 150 gr. bullet. The factory shows a velocity of 2390 fps, and the Ackley can skip along at 2700, even from common lever action rifles according to Mr. Ackley. That just happens to be faster than the factory .300 Savage, and yet it is the good old fashioned, century-old cartridge, improved.

Sixth best gain belongs to the 7x57mm Ackley. Since we have properly verified loading information in the Speer #2 Manual, those are the figures we used and thus the Ackley beats the factory 7x57mm Mauser by 12.8%. The 7x57mm Ackley equals the factory .270 Winchester and does so with 10 grs. less powder and less recoil. Ackley said this cartridge has the ideal capacity for a 7mm, and is powerful enough for all North American big game hunting. I have friends who use this one, and it is outstanding.

The seventh place round is another surprise to most shooters. It is the .300 H&H Ackley Improved, with an increase over the standard factory round of 11.1%. The factory loads the .300 H&H with the 180 gr. bullet to 2880 fps, while the Ackley gives 3200. There have been a number of improved .300 H&H cases, including the .300 Weatherby. They all headspace on the case belt, so the case body can be blown out to most any shape. Loads for the Ackley and the Weatherby are the same. Ackley always stated that these blown-out belted magnums are all badly over bore capacity and greatly inefficient compared to the standard H&H. Barrel life is short and faults far outweigh all good points, which seem to be limited to perhaps better case life when headspaced on the shoulder instead of the belt.

And now we arrive at number eight, the .257 Ackley with a gain of 9.4% over the standard .257 Roberts load of 2650 fps with the 117 gr. bullet. The Ackley gets 2900 fps with the same bullet. If we use the Winchester +P load of 2780 fps for the Roberts, the Ackley gain drops to 4.3% and the cartridge is not worth chambering for. This shows very plainly how poorly the factories have historically loaded the fine .257 Roberts.

This takes care of the best eight Ackleys, in order, compared only to standard factory loads and not with all bullet weights available. And we must remember that every barrel gives different velocities, so those listed here may vary from what other experimenters might achieve.

Now we will go to the other extreme, the poorest of the Ackleys. Ackley tested many cartridges that he knew before he started would show poor results. But with so many shooters asking about them, he felt obligated to prove just how inefficient they might be. And in some situations the Improved case actually showed zero gain! A good rule of thumb concerning Improved cartridges is that unless the gain reaches a minimum of 6% the project is not worth doing. Loading books often show peculiar numbers, some even appear to be incorrect. An example would be when a book shows a top load for a heavy bullet and then reduces the same powder when loading a lightweight bullet. Since a lighter bullet can always utilize more powder, that book value simply cannot be true. That is why experimenters with chronographs find such interesting things…. Of course, there could be extenuating circumstances that the book did not mention, but should have.

The worst of the Ackley Improved cartridges has captured last place in the chart, number 25, and is the 6mm Remington Ackley Improved with the 100 gr. bullet. The factory lists 3100 fps, and the top load found for the Ackley is only 3200 fps. This is a gain of just 3.2%, and is surely not worth going after. Even the 75 gr. load shows a marginal gain, as we shall see.

From the bottom of the chart, holding down number 24 of the 25 listed loads, is the .257 Ackley Improved when compared to the.257 Roberts factory +P load of a 117 gr. bullet at 2780 fps. The Ackley only gains 4.3%, or up to 2900 fps. This is another Ackley round not worth chambering for. Some books require close reading because they list the standard .257 Roberts with a 22” barrel, while showing the .257 Ackley with a 26” barrel. Assume 25 fps per inch of barrel and you have to remember to either add 100 fps to the standard velocities, or subtract 100 fps from the Ackley readings in order to make a proper comparison.

Tied with the .257 Ackley with +P loadings in 24th place is the .260 Remington with a 140 gr. bullet factory listed at 2760 fps. The .260 Remington Ackley Improved sends the same bullet off at only 2880 fps, or just 4.3% gain. This is well below the rule of thumb of a 6% minimum gain before making a change to any particular Ackley. It also is one that Ackley did not design as such, but did work with as a wildcat cartridge back in his day, the 6.5mm-08.

The fourth worst is the .22-250 Ackley with a 55 gr. bullet at 3850 fps. The factory .22-250 gets 3680 fps, which means the Ackley only gains 4.6% over the factory round. Here again, if we abide by our rule of thumb when chambering for any Improved cartridge, we would have to pass on this one.

Next on our list is one not too many shooters have interest in, the .375 H&H Ackley loaded with the 270 gr. bullet. The factory .375 H&H sends that bullet off at 2690 fps, while the Ackley gets 2830 fps, a gain of 5.2%.

The sixth cartridge of the less than desirable Ackleys is one from a logjam of three, the 6mm Remington Ackley with the 75 gr. bullet, as mentioned above. Here we find the standard factory 6mm Remington sends off the 75 gr. bullet at 3400 fps, and the Ackley only gets 3600, for a gain of 5.9%. This is close to the 6% required by our rule, so some shooters might decide to chamber for this one. And this particular cartridge shows that some cartridges are better with one weight of bullet than with another. The 100 gr. in the Ackley is not good, but this 75 gr. is perhaps acceptable.

Second out of the three-way logjam is the .220 Swift Ackley with a 50 gr. bullet at 4100 fps. This is a 5.9% gain over the factory load of 3870 fps, and again, some shooters might go for it. But there are other Ackleys that are better, or at least more efficient. Historically, none of the Improved Swifts have done very well, including the .220 Weatherby Rocket of days gone by.

Third of the three-way tie is the 7mm-08 Remington Ackley with a 140 gr. bullet at 3030 fps. The factory 7mm-08 gets 2860 from that bullet and the Ackley gain is again 5.9%. It is noteworthy that this Ackley load outperforms the factory .270 Winchester with the same bullet weight, and does so in a short action.

Right on the borderline of our rule of thumb for getting involved in is the old-time .219 Zipper, no longer being chambered for. The factory load with a 55 gr. bullet is 3300 fps, and the Ackley can achieve a velocity of 3500 fps, a gain of 6.0%. This is a rimmed cartridge and just about obsolete, with very little demand.

This now leaves us with the so-called middle ground calibers. They are all in the acceptable or better range of Ackleys, but of course some may be a bit more desirable than others for various reasons. Some only make the list with one particular bullet weight, or they make the list twice in different positions due to bullet performance resulting in different percentages of gain. Not all bullet weights are considered. The chart will show details of this, so here we will only list the cartridge and the gain, from best middle ground to worst. In gain order, the .270 Winchester Ackley, 8.8%; .25-06 Remington Ackley, 8.4%; .375 H&H Ackley, 8.3%; .243 Winchester Ackley, 8.1%; .270 Winchester Ackley, 7.8%; .280 Remington Ackley, 7.7%; .30-06 Springfield Ackley, 7.4%; .30-06 Springfield Ackley, 6.5%.

As you can see, each of the Ackley cartridges has its own story. It would fill a fat book to even begin passing on all the stories, but maybe this information will be a start for you. The chart with this will show all these Ackleys and their gains in velocity. It should give your hot stove league plenty of grist for another long winter.

Keep shooting, and hopefully with one of those Ackley Improved cartridges.


Bob Jourdan
PS & TAR Staff
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Old 07-19-2003, 11:15 AM
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Kent, thanks for posting, but I'd suggest that next time just put a link to the article.

The folks at Precision Shooting may not mind in this case, but it's possible that someone might claim copyright infringement if an entire article is copied.

Links generally are not subject to copyright protection.

Like I said the PS folks would probably not care and may even appreciate the publicity, but not everyone in the world is as generous.
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Old 07-19-2003, 09:16 PM
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Interesting article. There is also an article in Handloader this month about Ackley cartridges which is not so enthusiastic.

I pretty much come down on the un-enthusiastic side. I have just enough education in physics to believe you can't get a huge increase in velocity from a small increase in capacity without a corresponding increase in pressure.

I noticed the PS article did a lot of picking and choosing of data. For example, they show the .257 AI getting only 2900 fps with a 117gr bullet, and pretty much say its not a worthwhile conversion. Yet Accurate Arms has pressure tested data that shows about 3000 fps with that bullet weight (and yes, there is a corresponding increase in pressure over the parent cartridge). I'm not sure how the author chose his data.

Do people who shoot Ackley cartridges get velocity increases? Sure they do. But it is in large part due to higher pressure. Tests with pressure barrels show that velocity increases at about 1/4 the rate of case capacity if pressure is held equal. An 8% increase in capacity gets you about 2% in velocity.

Ackley didn't have pressure testing equipment.

The best thing about Ackleys is they don't stretch much.

I wouldn't want to discourage anyone from getting one, but don't think they're magic.

PS- sorry to sound negative, but I have to say the article kind of rubbed me the wrong way. To disparage the .257AI (which was one of Ackley's favorites) and then say the 250 Savage AI can beat a 25-06 with 100gr bullets just doesn't sit right. My .02.
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Old 07-20-2003, 06:30 AM
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I agree, that to say the 250 Savage AI can beat a 25-06 with 100gr bullets, casts doubt over the rest of the article.

I agree that there is no free lunch with the Ackley improved cartridges.


My most accurate rifle is a VZ24 Mauser in 257RAI, and is my only improved rifle.
My second most accurate rifle is a Ruger #1 in 223.
Would I rather the 257 was just a Roberts or the 223 was a 223AI?
Neither. The gain or loss from an AI is not as big as the effort of new dies, loads, and formed brass.

Fire forming brass to an AI chamber is a big pain, and trimming brass from a non AI chamber is a big pain.
Choose your pains.


The lack of bolt thrust is a novelty. Ackley's story about a lever action with no lock on the chamber, just the lever holding it closed is interesting reading, but does not mean much for a '98 Mauser.

Getting a 200 fps increase might mean allot with a 30-30 or 25-35, but with a 257, it is academic.
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Old 07-25-2003, 08:18 AM
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You can start quite a discussion with the simple mention of P. O. Ackley and Improved cartridges.
No one who has looked at past posts on the Beartooth forum would have been surprised at the efficiency of the .30-30 Improved. Excellent past posts by Terry Murbach and others accurately describe their results. The enthusiasm of their posts is infectious!
Don’t underestimate the benefit of reduced back thrust caused by the straight wall of the improved cartridges. This benefits both the bolt and lever action rifles. The Pressure chapter in Volume 1 of Ackley's Handbook for Shooters and Reloaders has a good discussion of this.
P. O. Ackley and others of his time sold high performance. Two excellent articles from past American Rifleman magazines help explain their goals. The first article is from the December 1953 issue. The H. P. White Laboratory tested the .30-06 Improved. Loaded to standard .30-06 pressures they discovered no "improvement" and William P. Wieman entitled his article: "How Improved is It?"
P. O. Ackley answered his critics in the October 1954 issue of the Rifleman. His article, entitled: "'Improved' Cartridges" states his thoughts very clearly. This article contains one of my favorite "gun writer" quotes.
"At the beginning of these comments certain things were pointed out as possible improvements, but nothing was said concerning pressure. Whether the handloader or wildcat enthusiast is right or wrong, he is interested in several things, most important of which are increased velocity and whether the bolt stays in the gun. If he can achieve these two results without serious complications, he is not entirely concerned with its actual pressure readings in pounds per square inch."

Another very good one:
"From the handloaders point of view, the amount of bolt thrust is of great importance. The pressure transmitted to the walls of the chamber can be safely contained by the use of high tensile strength steel in the barrel."

Taken out of context, you could come away with a mistaken impression of P. O. Ackley and his thoughts on high-pressure cartridges. Ackley understood the construction of rifle actions and the limitations of the brass cartridge case. A serious study of his writings reveals a very thoughtful, deliberate experimenter.
The second quote says a lot about my favorite rifle/cartridge combination, the .307 Winchester and the M-94 Big Bore rifle.

If you are unwilling to accept higher pressures with wildcat cartridges you will not see an improvement. Establishing reasonable limits, and sticking to them is part of the exercise. For many of us forming the cases and developing the loads is just as much fun as the trip to the range to prove the theory. The study of internal ballistics is a math exercise that will help you pass many evenings far away from the television set!
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Old 07-25-2003, 08:54 AM
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William,
Thanks for that thoughtful post.
My biggest complaint about Ackely is that he didn't write more.
I have read and re read the section on pressure many times.
I wish he had tested a '96 Swede.

I was not aware of the Ackley artictle.

I have had the same thought as Ackley in your quote, ~pressure doen not matter, the effects of pressure do~.

When I got into a 257 RAI, it was because someone else was buying the reamer, and loaned me the Ackley books.

I am very happy with the accuracy at the range with single shot, but I have yet to try to work on the feeding in a '98 design.
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Old 07-25-2003, 09:28 AM
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Kent,

I wish he had tested the Swede too.

Your thoughts on pressure hit the spot! For most of us velocity is not everything. Improved performance to achieve a goal is what we are after.
I load my .307 to just over 2300fps with 170 grain .30-30 bullets. For stand shooting I use the Sierra 170 grain flat nose. The Sierra bullet has a relativly thin jacket and opens up very quickly at this speed. For walking around in the short ceader I use the Speer or Hornady 170 grain bullets due to their stronger jacket. All three bullets exit, but I use them differently.
With my .30-30 Improved's I wanted to equal this performance in a 20" barrel, within reasonable pressure constraints. This was easily ackomplished with todays powders. Used lever action rifles chambered for .30-30 are readily available. Not so with the .307. I dont need the full potential of the .307 for the little white tail deer that I hunt. I like to walk them up like quail though and a little extra penetration is welcome when the deer is in motion on first sight.The mild recoil of the .30-30 Improved and the .307 helps you stay on target for a second shot. I have seen a number of 7 X 30 Waters lever guns out here for all of the same reasons.

For more P. O. Ackley reading check out the 1950's vintage Guns and Ammo. Do not overlook Robert Hutton's writing in the same vintage G&A's
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Old 07-25-2003, 10:54 AM
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I lost my mind on that last post and forgot to mention Charles S. Landis book: Woodchucks and Woodchuck Rifles. This was printed in 1951-I think. I wish the NRA's Firearms Classic Library would reprint it. Good copies of this book are expensive!
There is an interesting chapter written by P. O. Ackley which deals with wildcats, rifle actions and special purpose bullets, all combined with an interesting Colorado rock chuck hunting story. Well worth the effort to request as an interlibrary loan.
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Old 07-25-2003, 11:01 AM
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There's a Swede 96/38 in my gun vault that has been customized to 6.5x.257 AI - as sweet shooting a rifle as has been developed. There is about 100 - 150 fps increase over the standard 6.5x55mm cartridge across the board of bullet weights.

Used to have a Ruger M77 Ultralite in .257 Rob'ts AI, but a friend that couldn't keep his hands off it managed to talk me out of it. He uses it for coyotes at 300+ yds and always has a big grin on his puss!
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Old 07-25-2003, 07:12 PM
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Indeed, the AI Cartridges are great fun, and practical wildcats as well. I own several from .223, to .30-30, .30-06 and .375 H&H and others in-between! No, some of them don't turn up much worthwhile extra horsepower, but they all stop case stretch, and in the .30-30 and .375 H&H the benifits ballistically are way out of proportion to their paper calculated results!

Another wrinkle in the handloading enthusiast's wrapper, and yet another nook and cranny to explore on long, cold northern winter evenings!

Enjoy them all, they're great fun!

God Bless,

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Old 07-26-2003, 06:09 PM
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I see in today's flyer from Midway that Lee makes some seasonal dies for $23.98, including the 223 RAI, 22-250 AI, 257 RAI, 6.5-300W, 280 AI, 30-30 AI, 338-06, and 416 Taylor.

I already have Forster 257 RAI dies, but I have yet to rebarrel/ rechamber my '94 30-30 or '99 303S, and the 30-30 AI and 25-35 AI are high on my wish list.
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Old 07-28-2003, 03:32 AM
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The .30-30AI has been an eye opener for me. A lot of fun. I have a 24" .25-35 barrel from the Fox Ridge shop. One day I will open it up to improved.
Now that Lee is making .30-30AI dies at a reasonable price the conversion is quite reasonable cost wise.
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Old 07-28-2003, 09:29 AM
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I would expect the 30-30 AI to be one of the better cartridges, since the shoulder is moved forward as well as the reduced taper.

On another note, I would like to see a test done on actual backthrust, since it is a mainstay of the Ackley concept. For example, take a standard and an improved round, load them to the same pressure measured by piezo equipment, and then measure bolt thrust some way. I envision a machine with a piston where the bolt head would be on a conventional rifle, however I wouldn't want it locked in place. Then measure the thrust on the piston. An alternative method might be to use a copper piston, and measure the amount of crush.

Anyway, call me a doubting Thomas, but I question how much thrust is reduced by a straighter case. Given how much I have seen cases stretch at the web in a rifle w/ excess headspace, I don't think the brass itself is strong enough to reduce backthrust much, provided the straighter case sides do "stick" better.

I still tend to think most velocity increase is from higher pressure (Ackley acknowledged this), and I think we get away with it because rifles are strong!

But I don't really know, that's why I would like to see it tested. Is anyone aware of such a test, carefully and scientifically done?

Again, I don't want to knock the Ackley rounds, I'm just not as enthusiastic as some!
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Old 07-28-2003, 09:51 AM
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Me too - I think that you could just glue one of the pressure sensors like the Oehler M43 uses over the locking lugs and measure stress on the receiver. Not as exact BUT a useful 'apples-to-apples' comparison without too much cost or modification to the gun.

For what it's worth - brass stretching can absorb a considerable amount of backthrust. I was skeptical too till one day I was bored and found out the tensile strength of some common brass alloys, then figured out the cross-sectional area of some rifle cases. Can't recall the exact figures but it was a substantial percentage of the total calculated backthrust, best I recall between 10 and 20%. Hopefully I did the math correctly.

Brass is stronger than you think, some of the better alloys will yield at over 40,000PSI. Of course it's pretty thin in a cartridge case and has to be supported properly. Not sure how cartridge brass fits in to these numbers but the numbers really surprised me. Will have to look up the numbers and re-run the calculations, and post.

It's not totally cut and dried because once you pass the yield strength of the material, the amount of force needed to continue stretching the material can drop off (for one thing the brass is now thinner). And there is a time element involved because everything doesn't happen instantaneously. The stretching (or compression of the bolt lugs if you will) is happening over a period of time so the load on the bolt doesn't happen all at once, or even necessarily top out at peak chamber pressure. Plus - a huge variable is the amount of friction between the chamber and the case, hence the warning regarding oil in the chamber.

On the bolt-backthrust issue as a result of taper, well, if you've messed with any machine tools that work hold only by a taper, it will make more sense. The tapers hold very tightly till there's the least bit of separation, then there's no strength at all.

Last, there is sort of a chicken-and-egg aspect to the problem. When you reduce bolt thrust, you reduce case stretching - which means the cases are stronger and hold down the bolt thrust to an extent, and the cases are less likely to rupture even at somewhat higher pressure. Now... if one of them does let go, you may wreck the action when a round with 'normal' pressure would not have.

Like I said at the beginning I'm all in favor of some scientific measurements.
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Old 07-28-2003, 10:29 AM
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OK, some of this is from memory so bear with me -

Area of a circle = Pi * (radius squared) or Pi * r * r

Radius of a circle being half the diameter of course.

Pi being approx 3.14 for these calculations. Note I don't know how to display scientific notation like 'squared' in html so this will look a little confusing.

So, calculate max bolt thrust on a .30-06 sized cartridge with the case head diameter being about 0.473" and the max pressure being as high as 60,000PSI and case wall thickness of about 0.020" at the point it is stretching:

Maximum cross-sectional area of the inside of the case. Note that case walls are around 0.020" or so above the web:

((.473 - 0.020 - 0.020) / 2 )(squared) * 3.14 = 0.147 square inches.

Max bolt thrust at 60000 PSI = 8,831 pounds

Cross-sectional area of a case with max diameter of 0.473 and wall thickness of 0.020". Get the area of the chamber cross-section, then subtract out the cross section inside the case:

(.473 / 2)(squared) * 3.14 = 0.176
0.176 - 0.147 = 0.028 (I'm rounding so answers aren't coming out exactly correct but the final number accounts for all significant digits).

Assume brass has a yield strength of 40,000PSI (and this is where I'm going from memory), then the amount of backthrust needed to stretch the brass is:
40,000 * 0.028 = 1138 pounds.

total backthrust reduced by force needed to stretch case:

8,831 - 1,138 = 7,693 for a 12% reduction - assuming the friction between the case and the chamber wall prevents the case from moving back without stretching!

12% doesn't sound like a lot, but then consider that the 12% reduction in backthrust should get a 12% reduction in the compression of the bolt lugs. So the case stretches even less.

Like I said some of the figures aren't exact due to rounding but Excel did the math to a lot more decimal places than you see here.

Now, thicker brass changes this equation, as does a different value for the tensile strength of the brass. So does case diameter! Go to a smaller case (like the .30-30 or even the .223) and if the bras thickness is the 0.020" or so, the percent reduction is a LOT more.

On the big belted magnums it might be less of a reduction.

I'll have to look up the strength figures on brass tonight and also re-measure some of the cases I have sectioned.
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Old 07-28-2003, 10:39 AM
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Mikes chicken and egg theory is probably pretty close.
Ackley in his chapter on pressure removed the locking lugs from a M-94 and fired factory .30-30's in the Improved chamber. Ackley did not say how he held the lever up, rubber band, tie wrap etc, but he must have somehow. The action did not open up. Ackley then oiled the chamber, and then unscrewed the barrel a thread or two for subsequant shots. Each time the action did not open.
I have wondered if plasti-gage, the plastic measuring strips would some how work to measure back thrust. Or, you could glue a transducer to the top of the bolt on a Winchester and measure the flex of the bolt. It might be necassary to make up a testing cap like Dr. Mann used, with a pull string and firing pin.
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Old 07-28-2003, 11:40 AM
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MikeG,
One problem with that calculation is picking where the case will stretch.
If the co efficient of friction [case to chamber] were high, then it would be at the rear where the brass is thick. With oil, the line moves forward, and from Akley's picture of a separated oiled case, it looks like an inch from the rear.

I know that they make some springs from brass from reading "Machinery's Handbook".
When I take 45acp cases out of a .469" carbide die, they spring radially back to .471".

"Handbook for Shooters and Reloaders" Vol. 1 1962 P.O.Ackley
Chapter 7 "Pressure" page 140
"To further illustrate this principal, and old, discarded, beat up Winchester Model 94 rifle was resurrected from the junk pile. The barrel was rechambered for the "improved" 30-30 with a 40 [degrees] shoulder and minimum body taper... [Ackley went on to get case separation with factory 30-30 ammo in a an oiled improved chamber with the barrel unscrewed 2 turns, but with a dry chamber the case formed but did not separate]"
Chapter 7 "Pressure" page 147
"To further prove the point, the locking lug was removed from the action entirely leaving the breech block or bolt with no means of support other than the finger lever. See illustrations. The rifle was fired several times with the barrel tight. All cases appeared to be normal except for excessive primer protrusion. Now to state that this action will handle only certain pressers, or that the brass case will not support any amount of pressure seems to be out of order. There is plenty of room for further testing along these lines but the tests described seem to indicate that a very small percentage of the CHAMBER pressure was transferred to the breech bolt in the form of thrust. In this test the barrel absorbed the pressure while the action merely furnish the means for detonating the cartridge except when the chamber was oily.

The other illustration shows a Model 94 which was accidentally blown up. The load was supposedly 16 grains of Unique behind a 150 grain case bullet. A double charge was inadvertently thrown with the results shown. The action was not damaged except for threads being expanded when the chamber section of the barrel split. The barrel separated completely just forward of the chamber, the bullet passing downward severing the magazine tube and blowing the forearm to pieces. The shooter was not injured. This shows the high pressure could not be contained by the barrel but hat relatively straight standard .30/30 case did not back up against the breech bolt enough to harm ay of the action parts. The receiver has since been replaced and rifle works perfectly"

As I trancribed this passage from the book, I realized, as William points out, that the issue of compliance has been ignored here. The force to accelerate the finger gaurd and soft fingers is different than a stiff breech block. Not to say Ackley is wrong, I can just imagine an out of control variable.

Also, the issue of timing has been ignored here. Blowing up the barrel will reduce pressure before it can finish affecting the action. The pressure in the chamber is rising, not static, so the pressure is not the same in different places. Not to say Ackley is wrong, I can just imagine an out of control variable.

I have looked around this morning for 25/35 AI die set prices, and they are STEEP. The 30/30 AI with affordable dies looks better.

Last edited by Kent; 07-28-2003 at 12:03 PM.
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Old 07-28-2003, 12:26 PM
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Kent, you are right, my calculations are not 'absolute' or the final word on this.

Just an example, when we assume some things - like in this case I assumed most of the stretching would occur at the web and that the brass was 0.020" thick at this point. What I was trying to show is the maximum amount that the brass case helps to reduce bolt setback.

I've seen some odd case separations including one that started at the rim and went forward! This on a once-fired (military) .223 and it might have just been a faulty or over-hardened case. It was not an overload either. Wouldn't want to try to calculate the effect on bolt setback on that case - don't even know where to start.

The load on the locking lugs of any gun is much less than what is on the inside of chamber - since the area of the case head is much smaller than the rest of the area in the chamber. In the case of the .30-30 it is even less so than with the .30-06 as the area of the head is reduced considerably. That's the problem with making a blanket statement that 'lever guns are weak' as it may not be true at all, depending on the dimensions of the cartridge used.

Pressure on the locking lugs is only one of the loads on the gun, and it may not necessarily be the weak link.

Some calculations that I have seen in print (Precision Shooting) indicate a large safety margin in the bolt lugs for ordinary bolt-action rifles, in the magnitude of several times the ordinary max chamber pressure.

Obviously the barrels have some margin of safety, dependent on the quality of the steel (hardness), the thickness over the chamber, and any other built-in weak points like cuts for extractors. Usually this is a multipe of the expected chamber pressure also.

So.... it is quite possible that in some actions, the barrel might be the first thing to let go. In other actions, the bolt set-back might leave enough of the case unsupported that the gas flows back through the action and wrecks it, leaving the barrel basically unharmed. Or even just enough gas through a blown primer can cause problems.

The Ackley shape may buy some increased performance and even reduce the stress on the gun in one area, but may also cut the safety margin in other places.

I have seen pictures of several different kinds of gun failures. It is evident to me that there are few absolutes once the pressure gets out of hand - all sorts of bad things can happen and the results may not be predictable.

Yet another reason to not get carried away with our reloads...
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Old 07-28-2003, 12:44 PM
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I once read a description of Enfields being stretchy actions. I was suspicious it might have more to do with the wide range of acceptable headspace.
I went through the calculation of who much it would stretch with how much pressure.
That was fairly easy, as it is funtion of the steel size and shape and heat treat doesn't count until yield.
The action did stretch a lot, mostly from bolt body compression.

The problem was the brass.
How much stretching can the brass take?
What is the hardness of the brass?'
And after this discussion,
Where along it's length will the brass stretch?
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