Article: "Accuracy Testing: Shortcomings Of The Five-Shot Group"

Up until recently (speaking from a perspective of advancing age), the 3-shot group was the standard for evaluating the accuracy of a firearm (or shooter). But this is simply not enough data. So now a 5-group round is recommended. But, as this article from Shooting Sports USA observes, there are even shortcomings to the 5-shot group. An excerpt:

     Putting the gun in a mechanical rest, like the Ransom Rest, or putting the barrel in a barrel fixture can reduce the overall spread of group size since it eliminates human-induced error. However, group size still varies from one five-shot group to another with the same ammo. That is, no matter how tight the gun is or how good the barrel or the ammo is, all shots don’t go through the same hole on the target. How much variation is there? More than people might expect. Let’s look at some data.

     Table one shows data from 50 rounds of the same 9mm Luger ammunition fired in five-shot groups at 25 yards with the pistol in a Ransom Rest. The table shows 10 five-shot groups in the order they were fired. The smallest group (7) was 0.82 inch and the largest group (1) was 2.29 inches. As such, the largest group is 2.8 times larger than the smallest group.

      If we refer back to the previous example, we see that it would be premature to say that a 1-inch five-shot group from load number one was conclusive proof that it is more accurate than load number two which produced a 2-inch group. The bigger group was only two times larger than the smaller one, and we now know that different five-shot groups with the same ammo can produce a larger difference than that.

     Here’s another statistic from that data—the aggregate group of all 50 rounds was 2.59 inches. Thus, all 50 rounds produced a group that was 3.2 times larger than the smallest group and 1.1 times larger than the largest group.

The author goes on to explain that small groups are a matter of chance, and your best measure of accuracy will be the largest group. Anyway, a lot more there, so read the whole thing.