The mystery of grouping to the left & right:
by B.B. Pelletier
This post was inspired by Ozark, who wondered why his groups were going to the left and right of the aim point. What he either didn't say, or I didn't pay attention to when he said it, was that each group was shot with a different pellet. That's normal for different ammo, but sometimes a gun will shoot the same pellet to one side or the other, and that's not right. Today, I want to talk about that.
Why do they do it?
There are two principal reasons for this phenomenon. The first is that the scope axis doesn't align with the axis of the bore. You took my advice and sighted-in at 20 yards, but when you did you noticed something strange. I told you that if you were sighted to strike an inch below the aim point at 10 yards, you would be approximately dead-on at 20 yards. You were for elevation, but not for windage. Your pellet struck 1/2" to the left of the aim point. Oh, well, no problem, you thought. You just adjusted the scope until it hit the aim point and figured you were done.
You were done for 20 yards; but when you went out to 30 yards, your pellet was 1/4" to the left again. It was right on for elevation but not for windage. And, at 40 yards, it's 3/4" to the left. What gives?
What's happening is your scope is not looking along the same line as your barrel. At distances closer than 20 yards, the pellet will strike to the right of the aim point, besides striking lower. At 20 yards, it's right on. Between 20 and 30 yards, it will strike right on for elevation; but after 20 yards, it will start moving to the left (windage). The pictures should make it clear.
The scope is looking in a slightly different direction than the bore of the rifle.
This is what it looks like on paper.
Believe it or not, some pellets spiral as they travel downrange! On a sunny day at the range when the sun is at your back and the distance is 50 yards or more, you can see the pellet fly downrange through the scope. Many shooters, including me, have seen pellets travel in a spiral path.
I don't mean the pellet is spinning on its axis, which is caused by the rifling, though that is also what causes it to spiral. I mean that the pellet is traveling in an ever-increasing circular motion, and it circles in the same direction as the rifling. The center of the spiral is not inside the pellet. If the rifling was right-hand twist, the pellet goes downrange spiraling clockwise. If a left-hand twist, counterclockwise. The pellet only does this when it is not stable - including situations of over-stabilization.
Pellet travels in a widening spiral.
Groups from a spiraling pellet will be both to the left and right of the aim point, so it is more difficult to pin down than the scope/barrel alignment problem. It doesn't just travel left to right or right to left. It goes back and forth several times as it flies downrange. I have no proof, but I think that pellets that do this eventually fly off in some direction and do not continue the spiral. They are unstable throughout their flight. At close range, they are so predictable that you can even zero a rifle and use it. You'll get tight groups, but they'll just migrate around the point of aim at every distance except the distance at which you zeroed the rifle.
One last observation. Pellets that spiral are also falling throughout their flight, so despite my illustration showing them climbing higher at a farther distance from the muzzle, the axis of the spiral is on a downward slant. They don't really rise as I have depicted here.
Those are the reasons for pellets that move side to side. You can test for this by setting up tissue-paper targets aligned with a laser and collect the targets to analyze what the pellets are doing in flight, but all you really need is a sunny day at the range with the sun at your back and about 50 yards of distance. With the right pellets, you'll see it for yourself.