Rifle Accuracy: The Mystery of the Moving Groups
These past three weeks, despite single-digit cold and gale-force wind, I’ve been shooting a heavy-barreled tactical .308 in an effort...
These past three weeks, despite single-digit cold and gale-force wind, I’ve been shooting a heavy-barreled tactical .308 in an effort to make it cough up five-shot half MoA groups. And the sumbitch fought me every step of the way. However, through clean living, I finally prevailed.
In the process, I noticed a phenomenon that occurs very often when working up a handload. The groups moved to different locations on the target. Up, down, right, left, usually an inch or an inch and change. Sometimes they’re nice, tight groups; more often they’re three shots together and two a half-inch away, or four together and one flyer. Every once in a while you’ll get five through the same hole, but then things go back to being erratic.
There are a number of factors that can make bullets move: Wind is the most obvious, but none of the shooting was farther than 100 yards, and I was firing 175-grain Berger OTM (Open Tip Match) Tactical bullets, a projectile with a BC of .510, which is very high, and they don’t get pushed by the wind very much, if at all, at 100 yards. So you might see an out-of-place hole in the target once in a while, but not a whole group.
Or you can shoot a barrel hot enough that the rising heat waves create mirage, which gives you a false image of where the target is, and you’ll aim high. But it was much too cold for that.
Or, you can shoot a barrel hot enough that it will cause it to warp. However, this was a #7 contour barrel that I could not cause to warp on a 90-degree July day with a 20-round rapid-fire string, much less a 20-degree February day, firing 5-shot strings slowly.
So, there’s one other cause, and as Sherlock Holmes observed, when you remove all other causes, the one remaining is the culprit. And that is velocity, and pressure curve, that are not quite right to stabilize the bullets.
With this rifle, it turned out that the correct speed is 2,530 fps. I had been pecking all around it, and had, in fact, achieved it with RelodeR 15 and IMR 4320, but both those powders burn too slow, and when I switched to IMR 4895, which is a couple of clicks faster, I hit the right velocity and the right pressure curve, and everything went into the same ragged hole, and the groups did not budge from where they were supposed to be.
I went to IMR 4895 because I understand that it was the original choice for loading the M118 7.62 sniper round when the military went to 175-grain Sierra Matchking bullets. The idea was to create a load that would remain supersonic at 1,000 yards, and rather than looking for more velocity, the people in charge dropped the then-standard 168-grain bullet and went to 175 grains at a muzzle velocity of 2,570 fps.
And a side note: Berger bullets are more fashionable than Russian conspiracy theories, and can be even harder to find than actual proof of same. In a frantic on-line search for the 175-grain OTMs, I happened upon Graf & Sons, who have big bunches of Bergers, including the ones I was looking for. They are very, very efficient, and I recommend them highly.