Biggest Influences on MV Standard Deviation

[Chanonry]

I did some quick testing of mv on some 308 loads yesterday. It was not statistically significant but the results set me thinking and I would appreciate some input.

I had loaded 155gr Scenars in Lapua brass with RL15; some 180gr SMK in Federal brass with RL17 which was a slightly compressed load; and some 168gr AMAX in Federal brass with RL15. All 210 primers. There are reasons for this odd mix which I will not bore you with.

Point was that the 155gr and 168gr has SD's of 19 and 13 while the SMK was 8. This compared with a commercial Federal load with sd of 30fps.

The test was too small to be statistically significant, but it got me wondering about what the key elements of consistency are.

I guess the physical dimensions of case and bullet will be fundamental, so quality components plus reloading consistency are important but what about powder, primers etc.

Are some powders just more consistent than others?

Are compressed loads an advantage?

Is any component variance masked by reloading variances?

Should I just get a beer and chill?

 

[Lindy]

Re: Biggest Influences on MV Standard Deviation

Well, you might want to have a beer, and read this:

http://riflemansjournal.blogspot.com/2010/07/statistics-for-rifle-shooters.html

I'll leave it to others to list what they believe are the factors in a low SD.

 

[Cascade Precision]

Re: Biggest Influences on MV Standard Deviation

While it is WAY too early for me to be drinking a beer (6am here), I did ponder my mathematical mind over that article.

I am happy to say that I have been vindicated by that article.

Thank you Lindy. Truly an excellent read. Yes, even at 6am, I understand all that article has to say.

 

[Lindy]

Re: Biggest Influences on MV Standard Deviation

"Beer - it's not just for breakfast any more..."

I tend toward Bloody Marys in the morning, myself.

 

[ChadTRG42]

Re: Biggest Influences on MV Standard Deviation

I have also done many similar tests. I find that a consistant and exact powder charge is the most critical to low ES and SD's. If you ever get a chance, pull some bullets on factory ammo, and weigh the powder charge. You will see a large variation in the amount of powder in the case. I have seen as much as .9 grain (yes, almost a full grain) variation in very expensive and high end ammo. When most people test for accuracy, they test anywhere from .2 to .5 grain increments (I test at .3 grain intervals). You will get around 15-20 fps more or less velocity with about .3 grains of powder. So, a few tenths of a grain will cause a variation in velocity. This is why I weigh my powder charges on a digital scale that is accurate to .02 grains. I get the exact powder charge I want down to 1 kernel of gun powder. If you are accurate to .1 grains like most scales, you are accurate to about 3-5 kernels of gun powder, which is ok. (But, I like being anal about the powqder charge)

The next important items would be consistant neck tension, same seating depth, consistant brass prep, and finding a sweet spot load for the rifle. When I find a sweet spot load, the ES will decrease and run on avaerage about 10-20 fps ES or lower. The other loads that are not the sweet spot load will run around 25 to 40 fps ES. It also helps to run a powder with the correct burn rate for the caliber and bullet combination you are shooting. Hope this helps!

Chad

 

[tatemarb]

Re: Biggest Influences on MV Standard Deviation

A wise man, owner of the local Irish pub, once told me that "you cannot drink all day unless you start in the morning".
I thanked him for his sage comment and promptly ordered my pint of Guinness!

tater

 

[tatemarb]

Re: Biggest Influences on MV Standard Deviation

Oh, and one thing that will affect ED/SD is component variation. For example case volume variation and bullet bearing surface. Most if not all LR comp shooters will sort components by weight, size, etc. Some take it to another level and sort bullets and cases using machines that detect internal variations or defects by spinning them at high speed. This won't affect ED/SD but will still affect accuracy.

tater

 

[Chanonry]

Re: Biggest Influences on MV Standard Deviation

Lindy

Thanks for this. Looks like I can have reasonable confidence that I have got my target velocities, but as I suspected my SD numbers are meaningless.

Andy

 

[Chanonry]

Re: Biggest Influences on MV Standard Deviation

Chad

I hadn't thought about the 'sweet' load delivering lower sd, but thinking about it, it must.

I too have measured powder from 'premium' ammo and it shocked me as much as the runout!

Andy

 

[Jerkface11]

Re: Biggest Influences on MV Standard Deviation

If your biggest extreme spread is 30fps don't worry about it. Unless you have a REALLY expensive chronograph it isn't that accurate.