How Many Groups to Measure Accuracy?

[denton]

The question keeps coming up, how many groups do you need to shoot, to know the accuracy of a firearm?

I didn't run the math for all possible scenarios, just for five-shot groups. The calculations also assume that you have removed all causes for things like stringing and shifts: Loose scopes, barrel rubs, etc.

The math is for groups measured using "extreme spread", or center to center distance between the two most distant holes.

For individual five shot groups, a groups as small as half of the long-term average, and as large as 1.5X the long-term average are common. Within that range, a single group is not giving evidence of real change of performance for the rifle, ammunition, or shooter.

If you shoot five five-shot groups, and average the ES, your estimate of group size can be depended on to about plus or minus 25%. If you do 10 groups, the result can be depended on to plus or minus 18%.

Example: You shoot five five-shot groups that average 1". You now know the true long term average from the firearm is between .75" and 1.25".

The actual percentages aren't quite symmetric... the plus percentage isn't quite equal to the minus percentage, but the numbers I've given here are simpler to remember and completely adequate for practical use.