SD

[JM4590]

Can someone school me on why when developing a load, the SD is crap for maybe the first 4-5 loads, and then BAM, single digit.

For example:
22 grns 18.9
22.5 grns 21.2
23.0 grns 15.4
23.5 grns 22.6
24.0 grns 7.0
These are 5 shot groups, and I read the the posting on number of shots required to have confidence in the actual numbers. Everything being the same except charge weights, what makes the numbers come down? Or is it just an anomaly that I need to shoot more shots?
Thanks

 

[nn8734]

You’re going to need more than just 5 shots per sample if you want meaningful SD/ES. 20 is bare min, 30 would give you ~95% confidence level that the sample’s SD largely mirrors the entire prod lot (however many rounds that ends up being).

Focus more on group size and center POI between adjacent charge weights instead of SD in the early part of your development cycle

 

[pabrousseau]

Also more consistent ignition and powder burn at the higher pressure.

 

[rangerlong]

Before I started using better brass and mandrel expanding, the only way I got single digit SDs was filling the cases or even compressing the charge.
Seemed like consistent neck tension was less important with higher pressures.