38 Special: Sort by headstamp, times fired, or not at all?

[Shrinkmd]

I guess it would also make sense if I get around to this project to rest the gun and try to measure some groups at 25 and 50 yards to see. Of course, I guess it depends which gun (38 vs 357) shooting them.

And we've all heard that improved ES and SD don't necessarily translate into improved accuracy, which is what we're after.

I have plenty of brass which is once fired (so says TJ Conevera, anyway) so the next time I fire up the press to load up my 38 target/practice rounds, I will sort out some headstamps and measure. Maybe 100 round batches.

 

[wanderinwalker]

Dump them all together, clean, load and shoot. I started by sorting by headstamp, but I ended up with 50 of these, 20 of those (and several hundred Winchesters). When I load pistol on a progressive I just use a quart-sized Gladware container to catch the finished rounds off the press, and they hold about 150-170 .38s. Life is simpler just keeping all of the brass together and not stressing about it. Scoop a bucket of brass, load until the bucket is full, go shoot.

Besides, I can't shoot a handgun quite well enough to tell the difference in accuracy.

 

[DickM]

Any people remember enough statistics to figure out the power calculation to determine what sample size I need to find a significant difference in ES or SD. Maybe a difference of 5 SD would seem worth finding?

A difference between means of 5 SD is actually very large, so you don't need many samples in each group to run your test if you only want to detect that large of a difference. Assuming that you're planning to use the traditional t-test with an alpha of .05, and a power of .8 (both of which are pretty standard), you only need 2 samples in each group if the true difference between means is 5 SD. That assumes normality and homoscedasticity, which are probably reasonable assumptions for the type of data we're talking about, and also the same number of rounds in each group.

Determining sample size by establishing a minimum difference in terms of SD is probably not the best way to go about this because I'm not sure how you decide what's an "important" difference in SD to test for. A better way might be to think in terms of the difference between the mean velocities of the groups. Is a true difference of 5 fps important? I'd say probably not. 50 fps? Yeah, that probably is. Let's say, for the sake of example, that we decide that a real difference between the two mean velocities of 15 fps is what we'd like to detect. For the power analysis, we also need to specify what the SD is, which can be approximated with a little testing ahead of time. As an example, I'm looking at some data I collected for factory 250 gr .45 Colt loads - approx. 700 fps (the actual velocity doesn't enter into the calculation, only the difference in velocity between the two groups) with an SD of 7.76, so let's use that.

So, if I want to detect a "true" difference of 15 fps, with an SD of 7.76, and the other parameter values discussed above, the required sample size would be 5 (in each group).

If you want to try some of these calculations, I use a program called PS, which is available on-line at no cost from Vanderbilt University. Here's the link: http://biostat.mc.vanderbilt.edu/wiki/Main/PowerSampleSize

 

[bamacisa]

For plinking don't sort by headstamp. ZERO BULLET CO. sells 38 reloads. Every box of Zero reloads contains mixed headstamps....works fine.

 

[Walkalong]

In pistols, some loads with big ES & SD numbers shoot great at close range, while some with better numbers do not. Numbers are not everything.