Std Dev question

[AmericanCarrigan]

Are there any other factors besides the consistency of the brass that contribute to low standard deviation? I'm trying to work up accurate .223 loads. Between two different factory loads and my reloads, I'm getting a similarly high std. dev. of about 25. SIG match grade gave me 15 for a 5 shot group. My 7mm Rem Mag loads are running about 10 ft/sec. std. dev. I'm using 69 gr SMK's and Varget with the 223, digital scale, CCI primers, Forster COAX press. My gun consists of an Aero upper and ARP barrel in 223 wilde.

Thanks

 

[Ned Ludd]

The small .223 Rem cartridge makes it more difficult to obtain low ES/SD if for no other reason than every bit of error involved has almost twice the effect on velocity as in a larger cartridge, such as a .308 Win case. Everything matters in the .223 Rem, including consistent neck tension, uniform case volume, consistent charge weight, etc. You might try sorting your cases by water volume (or use case weight as a surrogate). In my hands, case volume variance with Lapua brass can be good for as much as 30 to 35 fps extra velocity from the lowest internal volume (heaviest) cases in a Lot # of brass to the highest (lightest). Turning necks can also sometimes be an effective way to help minimize ES/SD in the .223 Rem, although I'd consider it a waste of time for an AR in most instances. Finally, I'd also guess that your rifle being a semi-auto doesn't help (as compared to a bolt gun). If possible, you might try an experiment where you measure velocity with the gas system turned off (if possible), in order to estimate any possible contribution to higher ES/SD from that source.

The bottom line here is that unless you're shooting 500-600+ yards, which I doubt you're doing with 69 SMKs anyhow, it's unlikely that [relatively] poor ES/SD values are ever going to be the limiting factor in precision with a typical AR. Many are simply not designed to shoot with the precision more commonly available in a bolt rifle. That is certainly not to say you can't obtain outstanding precision with an AR-style rifle - because you can. But it might take a bit more work to isolate the specific reason(s) for the higher-than-acceptable ES/SD.

 

[AmericanCarrigan]

Thank you very much for the reply. Right now I'm just trying to dial in accurate loads with the intention of shooting long distance in the future. I think I will at least sort/weigh the cases and see if that helps at all.

 

[wkdickinson]

Also, looking at the SD of 5 shots is pointless - the sample size is WAY too small to be statistically valid. IMHO a 25 shot sample would be the minimum for a reasonable measurement of SD and more is always better.

 

[AmericanCarrigan]

I'll be updating the data as I go. I think I'll try a few other powders as well. When I find a node, I'll shoot at least 25 to confirm the data. I'm wondering if a tighter chamber or another gun related issue might be a factor. Ill also play around with slightly longer COAL, and/or switch primers.

 

[rsmithsr50]

I have no issues with low es in 223..match loads in a match rifle natch AR rifle

 

[rsmithsr50]

do not get too wrapped around the number. it is statistics, and should be looked at in large sample size to be relevant. like 1000 shots. all else is trend data for us reloaders.

 

[Webster]

Are there any other factors besides the consistency of the brass that contribute to low standard deviation? I'm trying to work up accurate .223 loads. Between two different factory loads and my reloads, I'm getting a similarly high std. dev. of about 25. SIG match grade gave me 15 for a 5 shot group. My 7mm Rem Mag loads are running about 10 ft/sec. std. dev. I'm using 69 gr SMK's and Varget with the 223, digital scale, CCI primers, Forster COAX press. My gun consists of an Aero upper and ARP barrel in 223 wilde.

Thanks

You can shoot big groups with low ES.

 

[Bc'z]

And small groups with a high ES.
Rifles are individuals each has thier own traits. Don't get hung up on the #'s let the target tell you what shoots good and your individual rifle likes to feed on.

 

[AmericanCarrigan]

Right now my groups are pretty lousy, but I think that's my fault. So the only info I had was the chrono numbers, which seemed to have a high SD compared to my 7mm Rem Mag. I'm getting great groups with a 20x scope on my Sako, and poor groups with the 6x on my AR. I forsee a better scope in my future, or at least a swap to get this gun dialed in. So, I'm not worried about my groups quite yet. Maybe the SD is a moot point until I can see what the gun can really do.

 

[tomswede]

Also, looking at the SD of 5 shots is pointless - the sample size is WAY too small to be statistically valid. IMHO a 25 shot sample would be the minimum for a reasonable measurement of SD and more is always better.

5 shots are not enough, 10-15 is getting pretty close at 20 the SD number is good for our purposes. Can’t remember the small sample factor on top of my head.

 

[6milphil]

Take this for what you will. I have an Aero precision upper with an 18" ballistic advantage stainless barrel. 69gr smk's, varget, starline brass shoot under .75 moa but the sd/es aren't anything to write home about. Tried the exact same load with 69gr Barnes match burners, average velocity stayed the same, es/sd were cut in half but the group's opened up to 1 moa or a little over.