People talk about group size all the time, but how often do they actually think about what they’re measuring?
The simplest way to measure groups is to use extreme spread (ES). The vast majority of the time someone reports group size, it’s based on ES. Essentially, ES is the distance between the two most distant bullet holes in a group measured from center to center.
A much more complicated but informative way to measure groups is mean radius (MR). MR is complicated because you need to find the mathematical center of the group. From there, you measure the distance of each individual shot from the mathematical center and average them. Because MR is so complicated, it’s rarely used unless you’ve got a computer analyzing your groups for you.
MR is more informative because every bullet counts. In ES, only the two most distant shots count. Take this group for example, which consists of one bullet (green dot) that is five inches to the left of point of aim, and nine bullets (red dot) that miraculously all landed exactly at point of aim:
The extreme spread is five inches. If the red dot was just one bullet, or 99 bullets, it would still be an ES of five inches. If you put a hundred bullet holes in between the red dot and the green dot, it is still an ES of five inches.
But the MR is just 0.9″!
That’s because every shot counts, and when nine of the shots land very close to the target’s mathematical center the one flyer has much less impact on the group size. If the red dot was only one bullet hole, the MR would be 2.5″ giving us a diameter of 5″ … the same as the ES. That’s because now both the ES and the MR are counting all (both) of the holes.
Since we normally can’t — or won’t — do the math to calculate MR, ES has become the accepted way of measuring groups for most people. But this in turn means we need to pay attention to another important piece of information: the number of rounds in a group. Some folks incorrectly assume that a 10-round group is “better” than a 5-round group for measurements. But that’s not actually true. Remember, when we’re using ES we are literally ignoring all but two rounds fired. So as you fire more and more rounds to get your group, you’re providing more chances for one bad hit to skew all your results. You cannot know whether that one bad hit was the gun’s fault, the ammo’s, or the shooter’s. But nine perfectly landed bullets will still look like a horrible group if that tenth round is a significant outlier.
Therefore, you have competing interests. Shooting more rounds will have a better chance of showing you if there are inconsistencies, but one inconsistency out of a hundred rounds skews everything. How to resolve this? Shoot multiple groups. For example, I normally shoot five 5-shot groups. This provides a reasonable balance between putting enough rounds downrange to matter (25) while still counting the “data” from a significant percentage of them (10 out of 25).
Train hard & stay safe! ToddG
(thanks to the folks at mp-pistol.com for the discussion that sparked this post)