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  #1  
Old 02-10-2012, 04:54 AM
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Burn rate and Barrel length


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While perusing this forum I found a comment by Rocky Raab that seemed to debunk my assumtion that faster burn rate powders would perform best in shorter barrels(and vise versa) for a given cartridge. Upon some more searching I found this nugget from Unclenick that sums it up very nicely. It seemed like it should be brought out into the light a little more in a separate thread. The amount of knowledge shown here on this forum is amazing. Thanks to all those who take the time to share!

"On barrel length: Depending on the powder, in typical rifles peak pressure occurs when the bullet has moved somewhere between one and three inches down the bore. Just before reaching that peak, the initially ignited powder will switch over frôm progressive to digressive burning, at which time the remaining burning powder mass is around 20% or so. That number combines the tiny digressively burning pieces with the powder thrown forward with the bullet that never got started burning very well in the first place (the stuff that burns when you put a match to floor sweepings at an indoor range; not recommending you do that, btw—lead hazard). It burns in ever decreasing temperature and pressure, so it doesn't contribute much to velocity. Once the bullet has got maybe five or six inches down the barrel, you could stop burning the rest and would still get to about the same muzzle velocity as if you'd left it burning.

The result of the above is, until you get down to a barrel length under around five or six inches, the same powder that produces the most velocity in a long barrel will also produce the most velocity in a shorter barrel. Period. So, be clear that if you move to a faster powder for, say, a 16" as opposed to a 24" barrel, you are doing it to reduce muzzle blast and recoil. That can help accuracy, so it is often a good idea frôm that standpoint, but it will come with a velocity penalty."

Last edited by Fall Guy; 02-10-2012 at 05:27 AM.
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  #2  
Old 02-10-2012, 06:14 AM
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It's nice of you to try, but that burn rate/barrel length myth is more intractable. obdurate, and pernicious than crabgrass.
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  #3  
Old 02-10-2012, 06:55 AM
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Fall Guy,

I have learned more than a few things on these pages that corrected my understanding of ballistics, both internal and external. The myth about faster powders working better in shorter barrels is like the one that says on long-range shots you hold under for animals that are downhill but hold over for animals that are uphill. Physics tells us you hold under for both, but trying to convince some folks of that is nigh onto impossible. I suspect they're the same folks who think faster powders give better velocity in shorter rifle barrels.
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  #4  
Old 02-10-2012, 08:09 AM
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Geez Rocky! Expanding my knowledge of the english language as well as ballistics!
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  #5  
Old 02-10-2012, 08:53 AM
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I'm glad I didn't say that my prediction was refulgently vaticinatory!
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  #6  
Old 02-10-2012, 09:20 AM
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Thumbs up

In todays world of instant knowledge, instant transfer of that knowledge, and many more heads working to debunk "old wives tales", "stick it to the elders" and look askance at the old "unbased, biased and sometimes totally bogus" statements from yester year, it is no wonder that the "TRUTH IS OUT THERE"...and someone will discover it.

While burn rate and barrel length are "sometimes" co-evolved, case SIZE and powder AMOUNT also co-mingle and co-habitate with all the rest.

The problem being WAY to many "generalizing" statements get lumped into "specific" applications which may or maynot actually be true.

Once something is printed in some media, goes out into the ether and is absorbed by we humans, it becomes somehow "locked it stone" and true forever more.

NO amount of retraction OR correcting will change the initial statement...just look at the spin tactics of politicians, and how do you make truth out of a half truth.

All anyone can do is test the hypothesis IF they are inclined to wonder...otherwise the "fact" just rolls on and on.

Fall Guy...IF you are really interested in validating your observation...buy QL....it is full of the information you seek.
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  #7  
Old 02-10-2012, 10:40 AM
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Actually, that's one of the major points on which I disagree with Helmut Broemel (the author of QuickLoad.) The program has an output line that predicts percentage of charge burned before muzzle exit, implying that powder burns the whole length of the barrel. No other professional ballistician believes that, as far as I know - and I've asked a bunch of them.
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  #8  
Old 02-10-2012, 04:03 PM
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Exclamation

QL, like Load from a Disk AND Powley all use mathematical constructs, algorithms, to calculate the various factors. The level of accuracy compared to empirical testing depends on the level of "tuning" the algorithm achieves. All you have to do is to pump the same data through all three to see how much or how little they agree. I do it all the time...then compare that to all the manuals...I've NEVER seen any two manuals agree precisely.

The only way to know for certain...for a specific set of parameters is to actually test...basically by starting with a calculated 100% burn point and cutting the barrel off a specific amount and, again, either by capturing and testing the residue/gas volume or mathematically calculating the resultant from the velocity loss.

How many of these types of tests do you think a gun/powder maker does before extrapolating ANOTHER algorithm to use.

I think ballistics is understood well enough to believe the QL data is at least statistically accurate.

The powder DOES have to burn fully by a certain lentth point. The implication in QL is that there is a fairly specific point where there is a 100% burn on the powder...once the powder is fully burned there is still residual pressure that continues to accellerate the bullet untill the bullet either exits the barrel or the pressure drops below the point where friction overcomes the pressure, inertial takes over and the bullet starts to slow down...basically simple physics anyone can do if they have the proper tools.

QL uses a 95% point and a 100% point. To use the 338 WM, IMR4831/Barnes 225 TTSX data from a previous question, the 95% point for IMR4831 is ~10.8" and 100% at 26".

Punch in another powder and the points change....IMR 4320, a "faster" powder is 9.5" and 21.1" .

Go to 3031 and the points are ~6.6" and ~11.0".

All these numbers were generated at as close to the PMax of the 338 WM of 62,366 psi as I gould get without going over and were different weights.

It would seem that faster powders DO burn in shorter barrels...but that doesn't mean the various loads are more or less more efficient, achieve the best accuracy or what not...all it means is the powder burns in a shorter distance and produces a specific pressure and PMax with a specific bullet/weight.

This seems perfectly reasonable to me. That the points might vary due to many parameters is also perfectly reasonable to me as NOTHING in this sport is locked in stone.

I also think much of the human produced information has a certain bias to it...even the best, most learned particle physicist isn't alone and there is opinion galore when a few engineers get started on a problem.

Lots of opinions, lots of theories...may are true and many are not...more often than not the arguments ensue due to semantics and comparing apples to guavas or worse...and a whole lot of non-specific rhetoric.

AND...the fact that many of the questions are very hard to answer EXCEPT by specifics and no one can keep the "generalists" outside and quiet. Hahahahahahah

The data might be in a form unintelligible to some readers and we all read an understand through our own bias and prejudices.

All the various questions and answers keep me thinking and out of bars at least.
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  #9  
Old 02-10-2012, 04:28 PM
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I have always found powder burn rate need be tailored to bullet weight - probably sectional density but in a specific caliber it's normally the same thing - rather than to barrel length.

My work is primarily in handguns. For instance, a 'full charge' load of 2400 pushing a 158 grain bullet (of given property) will generate more velocity than a 'full charge' load of Bullseye pushing the same described bullet out of a six inch revolver. BUT, when we compare the same two loads from a 2.5 inch revolver, the 2400 load still gives more velocity.

Of course, there are 'costs'. The 2400 load requires more weight of powder. The 2400 load will give more muzzle flash.

On the other hand, barrel length has some effect on desired powder burn rate, but only in conjunction with chamber capacity as it control expansion ratio. Which is why H4831 is gangbusters shooting heavy bullets from a .30 caliber rifle and not such a great choice when loading .380 ACP rounds.

I must concur with Brother Raab; the myth of short barrels and fast powder is deeply ingrained in the imagination of the reloading community.

This is also why I believe a chronograph is a requisite secondary item for every serious reloader.
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  #10  
Old 02-10-2012, 04:38 PM
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Very cogent, NFG.

The reason why this particular myth is so deeply ingrained is due both to constant repetition and the fact that it is so easily and clearly visualized. It's just a shame that such a clear mental image is so dead wrong. It isn't far from the clear and obvious image that we stand still and the sun crosses the sky.
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  #11  
Old 02-10-2012, 04:45 PM
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The fact is, powder does burn down the barrel, and can be proven with the application of elementary math and the scientific method. If "X" equals the time it takes an ammount of powder to burn in an enclosed bomb. And "Y" equals the ammount of time it takes the same ammount of the same powder in a given cartridge to propel a bullet from rest at the end of the cartridge case down the barrel to its end. Then when "X" is greater than "Y" then at least some of the powder would have to be burning all the way down the barrel.

Last edited by OTTER; 02-12-2012 at 10:24 AM.
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  #12  
Old 02-11-2012, 05:33 AM
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Nope, in all respects. It's muzzle flash that convinces people that powder is still burning. But if you believe the Naval Weapons Lab knows anything at all about ballistics, Muzzle Flash should convince you that flash is something else entirely.
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Old 02-11-2012, 05:59 AM
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Okay, now it is official, my head hurts!! Lou
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  #14  
Old 02-11-2012, 06:39 AM
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You are most welcome, cvc944. I enjoy being felicitously avuncular.
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Old 02-11-2012, 06:49 AM
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Quote:
Originally Posted by Rocky Raab View Post
Nope, in all respects. It's muzzle flash that convinces people that powder is still burning. But if you believe the Naval Weapons Lab knows anything at all about ballistics, Muzzle Flash should convince you that flash is something else entirely.

Spot on, muzzle flash does not mean the powder is still burning
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  #16  
Old 02-11-2012, 09:38 AM
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Very true, Rocky, and very well understood by lawyers. Human observation and memory get us all into trouble.

ALL data is argumentative...again...because we mostly speak in generalitys about a specific set of parameters. The two are NOT mutually exclusive.

The good thing AND bad thing about the 'net is, as I've said many times, once the information is scattered about, there is no way to put the toothpaste back in the tube...it's worse than the Energizer Bunny...it keep's going and going and going.

But as time goes on and we develope newer and cheaper methods and tools the average reloader has access to, and reloader/shooters continue to evolve and become more and more "educated and well read", I think things will level out.

There are so many parameters to think about and optimize when doing your reloads/rifle building that it can overwhelm even advanced reloaders...and it all takes time and effort to hit that "sweet spot".

Just look what happened when chrono's became cheap enough so every reloader could afford one. Pressure testing equipment and "ballistic Labs" are almost in that catagory.

I bought the first Oehler 33 to hit the area way back when they had been out only a few years. Very few would believe the velocity results from their rifles. Now we are arguing about the nuances of cold effects on the batteries, which brand is the most accurate...etc...all the why's and wherefores of the resultant data, but NOT that the chrono has/hasn't any value in helping to produce accurate and repeatable data.

Everytime I build a new wildcat I basically use "last years" tecnology, because no sooner than I develope a "bugholer" load...out comes a new powder, or a new bullet, or some new information was developed and posted somewhere. I have a choice to stick with "the old" or checkout the "new". Keeps me awake at night...at least for an hour or two while I digest it. Hahahahahas

Times is achangin'...for the better.
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Old 02-12-2012, 10:28 AM
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Since I see the focus was misdirected by the last few words in my above post. rather than correcting them to say "increased muzzle flash" and leave the focus misdirected. I deleted them.
Thanks,Ron.

Last edited by OTTER; 02-12-2012 at 10:31 AM.
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  #18  
Old 02-12-2012, 05:46 PM
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Ron,

The challenge of being humble in the face of indisputable fact, contradicting what we have "known" for many years, is one that I too struggle with, from time to time. This website has humbled me several times! It can be hard to supplant what is intuitive with scientifically proven realities. You pull the trigger and flame comes out the end. Flame comes from stuff that is burning. What is burning in a rifle barre? Gunpowder, of course; so that must be what causes the flame coming out the end of the barrel. And the gunpowder DOES cause that flame, but as it turns out, it's not from individual grains that are still burning.

Consider this your one thing that you're supposed to learn each day?
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Old 02-13-2012, 06:03 AM
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And take comfort in the fact that we ALL have to overcome such "common sense" beliefs for things that seem completely counterintuitive. It starts with realizing the sun does not go around the earth...
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Old 02-13-2012, 07:09 AM
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And let's don't forget our previous clarification on this topic, which was along the lines of:
"All the powder is burned within xxx inches"
means
"All the powder that is going to burn inside the barrel is burned within xxx inches"

Nitrocellulose doesn't have quite enough oxygen in it to support complete combustion, so regardless of any other factors, there is going to be unburned fuel left once it hits atmospheric air. This fact alone adequately drives the above-referenced clarification. Nitroglycerin, an oxidizer, alters the potential chemical balance for double- and triple-base powders.
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