Posted by Mike G on February 13, 1999 at 20:27:53:
In Reply to: MEASURING A GROUP posted by BLAIR on February 13, 1999 at 15:30:47:
There are several ways of measuring group sizes. Mean radius is one of the most statistically accurate. It involves finding the center (centroid) of the group and then measuring the distance from centroid to each shot. MR calcuations null the effects of flyers in the group, so the radius represents the majority of the shots. I had a chance several months ago to evaluate some handsloads I made duplicating Federal's SEAL 300 Win Mag match load in an USN M40A2 sniper rifle. Since the military and ammo makers measure group size this way, I used the MR method for consistent comparative analysis. The MR method takes into account every shot, not just the outside two.
The Extreme Spread (ES) method mesures the distance center-center of the 2 widest shots, subtracting bullet dia. As mentioned in previous posts this is a common method in BR shooting, of which I have a device made by Custom Products Co. to measure group size accordingly. If ES has any disadvantages, it is that ES does not take into account the other shots in between your two worse shots not matter how good or bad they are. Nevertheless, for all practical real world purposes ES is a good method for determining gun, ammo and shooter headspace(between the ears) performance. Also by using simple formulas, ES can be used to determine MR of a group(s) and interpolating 3 shot data to equate to 5,10 etc performance.