11-10-2011, 04:00 PM
Wikipedia has a section of converting group size to MOA. Unfortunately I lost the precise URL but I've recreated the table.
The correlations are based on statistical calculations for the same rifle and ammunition combination (standard deviations of every shot from center is 1 MOA):
No of shots Size (′/MOA)
2 .................. 1.77
3 .................. 2.41
5 .................. 3.07
10 ................ 3.81
20 ................ 4.45
100 .............. 5.69
Remembering that we are looking at the standard deviation, we can still use this in our normal sense because the proportions are consistent.
So, a way to look at this in our context is to assume that we have a rifle that shoots 1 MOA 3-shot groups. That same rifle/load combination would likely give (5.69/2.41)X1.0 = 2.46 MOA groups when 100 shots are fired.
The scary thing is that a given 3-shot group can be anywhere inside that 2.46 MOA, so a sight-in based solely on a single 3-shot group will have a 2 or more MOA uncertainty in where the true zero is! Most of the time the 3-shot group will be near the center of the uncertainty circle because the shot dispersion largely reflects the bell-shaped curve.
So, do your shooting with 3 to 5 shots as is customary, but verify them by shooting several groups and averaging their centers for the final tweaks.
The correlations are based on statistical calculations for the same rifle and ammunition combination (standard deviations of every shot from center is 1 MOA):
No of shots Size (′/MOA)
2 .................. 1.77
3 .................. 2.41
5 .................. 3.07
10 ................ 3.81
20 ................ 4.45
100 .............. 5.69
Remembering that we are looking at the standard deviation, we can still use this in our normal sense because the proportions are consistent.
So, a way to look at this in our context is to assume that we have a rifle that shoots 1 MOA 3-shot groups. That same rifle/load combination would likely give (5.69/2.41)X1.0 = 2.46 MOA groups when 100 shots are fired.
The scary thing is that a given 3-shot group can be anywhere inside that 2.46 MOA, so a sight-in based solely on a single 3-shot group will have a 2 or more MOA uncertainty in where the true zero is! Most of the time the 3-shot group will be near the center of the uncertainty circle because the shot dispersion largely reflects the bell-shaped curve.
So, do your shooting with 3 to 5 shots as is customary, but verify them by shooting several groups and averaging their centers for the final tweaks.