May 22, 2009
Petzal: How Many Groups is Enough?
By David E. Petzal
I dearly love premium bullets, but shooting them these days is like firing Krugerrands down your barrel. So when I work up a load I make up three cartridges, shoot them, and if I get a worthwhile group I make up more and see how those perform. It would be nice to stop with three groups, but I’ve always been suspicious about shooting that few, and this past week I got an object lesson on why you need more to be certain.
I shot three groups from a very good .338 that averaged 1.118, and were all within a couple of thousandths of an inch of each other in size. They were even the same shape. When you get this kind of consistency as a rule, you can assume your work is done. But as a formality, I loaded up two more groups’ worth—six rounds—and watched appalled as they sprayed all over the target. I loaded another six rounds and again, it was like patterning a shotgun. For some perverse reason those first three groups could not be duplicated, and it was back to the loading room for something that did work.
You can, if you wish, rely on three groups to establishing accuracy. I know a number of very accomplished shooters who do. But I think the minimum number is five groups. The sweaty hand of coincidence can have its way with three, but not five. Play it safe; shoot up the extra ammo.