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I could use some help. I recently bought a used PACT chronograph. It works fine and gives wonderful information I'm sure, but I don't know what to look for. I'm trying to develope a load which is most accurate and consistent in my 222 Rem. What information should I be looking for in comparing different loads? Thankyou in advance for your assistance.
 

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copper

The best indication of consistancy of information from a chronograph is standard deviation.  The formula is frightening to look at but most scientific calculators have a calculator for it.  The easiest description of what it is follows.
 
The average difference of each observation from the mean of the sample.  A simpler explanation is the smaller the sd the more consistant the sample.
It also means that the smaller the sd, the more confident you can be that the next shot's velocity will be "close" to the average for the sample.

As you develop and test loads, as you start from the starting load and proceed to the max load you should see the Sd's of each get smaller until a point where the sd hits "bottom" then starts to go up again.  The "bottom" point is the most consistant load.  Note this may not be the most accurate load.  It may also be the max load.

A more technical description of sd can be found in Speer # 11 pg 466.
 

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Hi, copper:
Actually what dodgestdshift described is mean deviation or mean absolute deviation (MAD). Standard deviation is a bit more complicated. Average your velocities, then square the difference between each shot and the mean (average), add the squares, divide by the number of shots and take the square root of that.  You can do some further analysis with the standard deviation that you can't do with the mean deviation.

In theory, 2/3s (68%)of your velocities will be within one stanard deviation of your mean, 95% within two standard deviations and 99% within three.  

I worked out an example a while back, but that thread must have been lost when the server crashed several months ago. I'll do it again if you don't have a scienfitic calculator handy.

In some ways the extreme spread is more useful, since the bloopers and spikes are what you want to avoid.

What some folks call the ladder test is the best way to find a good load. Fire a series of shots at the same target as you work up a load. Likely the groups will move as you add powder. With luck, at some point they'll say put with another 1/2 grain increment. Your barrel is stable at this point and that's where you want to be. Now you may be well under the velocity you want and adding powder starts walking the groups again. In that case try a different powder or bullet weight. Sometimes another bullet of the same weight or primer or bullet seating depth make makes a big difference.

I like H322 in my .222, but there's one thing you should be careful about. There's been at least 4 versions sold. Surplus IMR 8208, Nobel from Scotland, early Australian and current Australian. If you look in several reloading manuals, you'll see 22.5 grains or 24.0 grains as MAX, with only Speer in between. Hornady #4 recommends 24.0 grains with their 50 grain bullets, but there's no way I can use that much early Australian in my old 700, even with V-Max Molys. The later Australian is slower, but I haven't pushed it yet.  

Bye
Jack
 

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"Bad Joke Friday" Dan (moderator emeritus)
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Copper,

Very good question. The primary two results I look at are the Std. Dev. which will give you your distribution as has been already nicely covered. The second criteria is the ES or Extreme Spread, which is nothing more than subtracting the slowest velocity from the highest velocity to determine the ES. As you can certainly guess, the smaller the ES, the better. But I no sooner say that then I refer to dodgestdshift's comment "Note this may not be the most accurate load."  I can't explain the magic (or whatever it is), but your smallest SD/ES may not be your most accurate load, but they are typically very close to being the best load and certainly are excellent indicators.

I would suggest one more function of your timing device that can be critical, and that is in determining your max. load. If, as is common, you start a few grains below the reference book's maximum load and work up, your velocity increases will be linear (not perfect, but fairly linear) to a point at which your velocity increases will significantly reduce or disappear. At that point, more powder will only dynamically increase pressure without velocity increases and you've obviously reached a maximum powder point. As I'm sure you've noticed, the reference books vary, and using the chrony to validate your particular rifle and components "point of diminishing returns" is a profitable use.

For a much better explanation on this important function, I'd sure suggest that one of your best buys would be the Beartooth Tech Manual.

Dan
 

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BTW, a 3 shot standard devation is useless, 5 isn't much better, 10 will tell you something, but a real statistican wants 33.

Bye
Jack
 

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Discussion Starter #6
Thank you all for your replys. If I understand correctly, I should be looking at the standard deviation of a set of 33 shots of a particular load. I know the chronograph will compute the standard deviation for me, and since I am mathematicly ( as well as spelling) challenged, this is a good thing. I will continue to experiment. What reloading techniques will improve my consistentcy? I load on an RCBS rockchucker for resizing, prime using a Lee hand press with ram prime, and weight charges on an RCBS scale (but will use the Pact electronic scale I bought with the chronograph from now on) and seat bullets using a Lyman challenger press. When seating the bullet I raise the ram part way, rotate the case a quarter turn,  raise the ram some more, rotate the case a quarter turn, and continue this until the ram goes all the way to the top. I was told this will help center the bullet in the case. Is this true? Are there any other techniques I should be using. Im not a complete novice, but I dont reload every day either.  Thanks again for all the replys and will look forward to your assistance.
 

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"Bad Joke Friday" Dan (moderator emeritus)
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Jack,

A "real statistician" would sure be disappointed with my efforts! I plan on knowing all I want to know about a particular load way before I get to the 33rd shot! But your point is well taken as I have noticed when I shoot/calculate five 5-shot groups individually, and then calculate the aggregate, the five groups vary from the aggregate somewhat.

Copper,

I'm afraid I'm not able to offer any additional suggestions on your techique, but I will tell you that my tests indicate that however hard I try, there is some inconsistency. By that I mean, I'll load and test one day, load with what I consider exactly the same load the next day, same settings, temperature, etc. and and the two day's results will vary. I'm not talking about accuracy, but the performance data we've been discussing! So as far as I'm concerned, you're on the correct path to think along the lines of good loading technique, because consistency isn't a given.

Dan



<!--EDIT|DOK|May 19 2002,23:34-->
 

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The Hog Whisperer (Administrator)
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Two things are very useful on the road to accuracy, in addition to standard loading gear:

One is a gage like the RCBS Precision Mic or Stoney Point, which measures how close your bullets are to the rifling when loaded.  This is a big variable and yet another way to tune a load.  Also helps you keep tabs on throat wear.

Other is a gage to measure case neck runout, like the RCBS Case Master (there are probably others too I'm sure).

Now.... it should be noted that you probably need to be consistently under 1.5 MOA, probably 1.0 MOA, before you have the basics down enough for these tools to help.  But they are very helpful in working out kinks in your reloading process.

Just putting loads together that are safe and will feed in your gun will occasionally result in spectacular accuracy.  But sometimes it takes a huge pile of components to get there, and not always possible to duplicate!  

These devices will also perform very valuable "Q-C" on your reloading dies.  I have found die problems by using those tools, that I might not have ever figured out.
 

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Hi, Guys:
 I shouldn't post so late at night. While what I said about the number of shots is correct, 10 shots is enough for practical purposes most of the time and 20 is the most I've used. I don't know the theory behind the customary sample size of 33, but I suppose it's enough at a confidence level of say 95%.  After all, I'm not a real statistician.

Alex was planning on a standard deviation calculator and some other statistical tests in Ballistician's Corner, but I suppose he's had to take time out for school.

 If you don't have a permanent indoor setup for your chronograph, you should check it with a .22 before you pop off the Noslers. I've heard that a good spring powder air rifle is even better.

 As for centering your bullets, give the case 1/2 a turn, not 1/4.  If it's cocked one way, you need a 180° turn to straighten it. You may want a concentricity gauge like the RCBS Case Master so you can find out what is causing any misalignment. In my case, the resizing die was bending my .222 necks, and the seating die was OK. If you've already got a dial indicator, you can make a gauge with a few odds & ends that you'll likely have already.

Bye
Jack
 

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"Bad Joke Friday" Dan (moderator emeritus)
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Copper,

Apologize for not paying more attention. MikeG's excellent recommendations alerted me that the questions pertain to rifle, not pistol, cartridges.

My remarks are directed to someone interested in increased accuracy (sub-MOA) for target purposes, not sporting rifles looking for 1.5" at 100yds.

1. Completely agree with MikeG's recommendation for gauges for determining the overall length for best accuracy. I use the Stony Point mechanism to determine bullet seating depth to result in the bullet olive being .003 off the lands. That improved my accuracy for my .223 in 14" Contender.
2. I also do something that is a time consuming pain in the neck, but it also made a noticable accuracy improvement, and that is I trim the necks to assure they have equal thickness.
3. As Jack and Mike point out, gauges to check concentricity and run-out also help. When I got started, I read everything I could get my hands on and then bought "a bunch" of equipment. Then I came accross an article that said that the primers, bullets and powder have pretty good consistency, but that 90% of the problem(s) was with the brass. The article concluded with the advice that you should consider buying the best brass in the first place, and every since then I've purchased either Lapua or Norma brass. In every instance, the concentricity, run-out and weight have all been within the benchrest recommended criteria. The better brass is expensive, about $54 a hundred versus the usual $15 a hundred, but it sure took between .5" and .75" off my 100yd. results without any additional efforts. And if reasonable loads are used, eight to ten reloads or more are common.

Dan
 

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Copper:

Another thing you can watch with your chronograph is what you might call powder efficiency (my phrase).  As you start at a low powder charge and gradually increase the charge, you will notice the velocity increases in the sample.  After you reach a certain point, which could be called the point of diminishing returns, the same increase of powder charge will increase the velocity by smaller and smaller amounts.  This charge where the velocity increases begin to drop is about where you get the most efficient velocity for your powder.
 
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