Thread: Testing a conventional approach to load development

Okay so I went and shot these this morning. Conditions were good - essentially nil wind, my usual setup shooting at 100m with Atlas bipod & rear bag prone.

First, I did the process @McNotty suggested - 2x 3rd groups, and a 4rd group. Here are the results -




1.861 ("best" load identified) - produced groups of
3rd 1: 0.57 MOA
3rd 2: 0.79 MOA
4rd: 0.55 MOA

Mean radius from these 10 shots overlaid was 0.101MRAD and a 10rd aggregate group size of 0.79MOA. Looked pretty good.

1.852 ("worst" load identified - produced groups of
3rd 1: 0.85 MOA
3rd 2: 0.54 MOA
4rd: 0.89MOA

Mean radius from these 10 shots overlaid was 0.099MRAD and a 10rd aggregate group size of 1.21MOA. Looked ... actually basically identical to the 1.861 load if you look at MR rather than group size. The individual groups weren't obviously different either.


So then a 10rd group of each. Looked like this. This looks like a really bad group with the "better" load - the 1.861 CBTO. However bear with me through the results.



Best load 10rd group size - 1.37 MOA
worst load 10rd group size - 0.92 MOA


So what do the 20rd from each load look like when aggregated to one big 20rd group ?

1.861:


Mean radius: 0.116MRAD group size 1.37MOA (4cm). The predicted O95 from the 20rd mean radius is 4.8CM so the group size measured falls within expectations. This is a worse group than I "expected" but it is within the calculated expectations of all the data. Did I fuck it up and pull a couple of shots - change cheek pressure - did the wind gust a little bit L-R ? Who knows but it is what it is.

1.852:


Mean radius: 0.09MRAD, group size 1.21 MOA.

It does actually appear if you look at the MR, that either -

1. There is no functional difference between the loads and there is just a bit of natural variation due to measurement error etc showing up in the results

OR

2: I cannot shoot well enough to resolve any difference between these different loads, and the difference in the MR values is the "noise" from my shooter error

OR

3. The load identified as the "worst" through 3-shot group testing is actually slightly more consistent over large samples than the load identified as the "Best"

I do feel that I may have introduced a little error on the 10rd group of the 1.861 load and, had I not, it would have likely produced a more or less identical result to the 1.852. The variance in the mean radius is higher than you would expect from natural variation but there is likely a bit of measurement error here - the group analysis in the app isn't perfect. Regardless the results are functionally identical and fall within the cone of fire of the system even if a couple right at the edge were maybe contributed by a little extra shooter error.


Here's all 61 shots with this powder charge, and all seating depths - a 1.37MOA group with a mean radius of 0.096MRAD





So conclusion - with this rifle, bullet, brass, powder and shooter combination it is not possible to identify any difference in precision between loads with different seating depths by testing with 3 shot groups, at .003 inch increments of seating depth (as recommended by Erik Cortina, if he isn't the person to listen to for this then who is ?). Or 20rd groups of same.

All results fall within the cone of fire predicted by the 10rd mean radius for any of the 10 rd groups for any load.

Writeup is a bit messy and rushed because I'm off hunting for a week