The mean radius method is nothing new, it's been the standard procedure for military ammo for ages. But it is rarely used by civilians because it's a pain in the butt.

So why use mean radius, if it is a pain in the butt? Because I try to

*things, and it's tough to prove a statistically significant difference using conventional group shooting. Three groups won't do it. Six groups are marginal. You really need at least 10 groups to prove statistical significance. That's a lot of shooting.*

**PROVE**Group shooting gives you only one data point for every group, but mean radius gives you one data point for every shot.

For example, if I shoot four 5 shot groups, that gives me 4 data points, not enough to prove anything. But if I take the same 20 cartridges and shoot two 10 shot groups and calculate mean radius, I get 20 data points, which is often enough to prove statistical significance.

Here's a link to a good explanation of mean radius. It's also discussed in Hatcher's notebook.

There is no exact formula to convert between mean radius and group size, but in practice, a 10 shot group tends to be 3 to 4 times the mean radius. For example, a recent session with a TC 357 rifle produced a 0.55" mean radius and 10-shot groups averaged 2.03", or 3.68 times the mean radius. If that ratio held constant, I'd need a 0.27" mean radius to shoot an MOA 10-shot group.

FYI I measure the coordinates of each bullet hole the old fashioned way (it helps if your target has a grid background) and then use a spreadsheet to crunch the numbers. I'm still tweaking the spreadsheet but if I ever get it perfected I'll share it, or perhaps convert it to a web page so everyone can use it. You can also buy software to help you measure mean radius.