Why does the point of impact shift from one side to the other?
by Tom Gaylord, a.k.a. B.B. Pelletier
This report is for blog reader Roger, who has this problem, and also for RifledDNA, who says he has it, too. Fred DPRoNJ (Democratik Peoples Republik of New Jersey), also expressed interest in the topic. I suspect that hundreds of our readers, if not thousands, are curious. Why would a scope that shoots to the left of the aim point at 10 meters be dead-on at 20 meters and off to the right at 35 meters?
Here’s part of what Roger told us:
“This time I’ve scoped a S-410 and something strange is happening. For example: I zeroed the scope for 11m, thus the far zero would be around 44m. When shooting that distance although the elevation is fine the POI shift to right around 7cm. If I re-zero the scope in 44m and shot back to 11m, although the elevation is fine the POI shift to left around 4cm.”
Roger doesn’t state the problem in quite the same way that I do, but it’s the same problem. Gun’s POI changes as the distance changes.
The answer is in the alignment of the scope with the gun’s bore. But before I get to that, I want to address an answer that someone else on the blog gave to Roger. Someone guessed that his problem was caused by spiraling pellets. Pellets sometimes fly on a spiral path as they travel downrange. I wrote about it in this report: Do pellets spiral?
That answer was correct, as far as it went, but there’s one way to diagnose the difference between spiraling pellets and an alignment problem. Spiraling pellets move back and forth from one side of the aim point to the other and back again as they go downrange. Pellets shot from a gun with an alignment problem do not. They’ll start out on one side of the point of aim and move to the other side as they go farther from the gun, but they’ll never come back. It’s a one-way trip for pellets shot from a gun that has an alignment problem.
Don’t blame scope shift
Too many airgunners are willing to blame problems like this on their scopes. They call it scope shift. Real scope shift is rare, though not unknown. If you want to learn more about it, read my article about scope shift.
Scope and barrel must be aligned
The scope and barrel must be pointed in the same direction for the scope to work perfectly. Because the scope has some latitude of internal adjustment, it’s possible for the scope to be out of alignment with the barrel and still have them coinciding at one distance. That distance is called the sight-in distance.
But if they’re not aligned, the shots will be off to one side when they’re closer than the sight-in distance and to the other side when they’re farther away. The drawing below illustrates what this looks like, although I’ve exaggerated the offset so you can see it. In reality, both the scope and barrel may appear to be perfectly aligned.
The axis of the scope (at the bottom on the right) and barrel are offset in this drawing, but the scope can be adjusted so they coincide at one distance. Any closer or farther away than the sight-in distance, though, and the pellet will move to one side of the other. This drawing is grossly exaggerated to highlight the problem in a short space.
How to correct it
To correct this problem, the scope must be positioned on the rifle so its axis is parallel to the axis of the bore. This may be easier than it sounds. If you have 2-piece scope mounts, for example, you can turn each of them around backward (one at a time, of course) to see if that corrects the situation. It often does. You can also swap the front ring for the rear ring for further improvement. There are 4 different possible combinations of scope ring positions with 2-piece mounts (scope rings). If you have 1-piece mounts, all you can do is turn them around backward. That’s why I prefer 2-piece mounts over 1-piece mounts in most situations.
If changing rings isn’t enough to correct the problem, you can shim them. Shimming the rings for sideways correction is the same as shimming for elevation correction — you’re just moving the scope in a different direction. A word of caution — when you shim the rings for sideways adjustment, be careful not to extend the shim too low on the ring; because if it gets under the scope, you’ll cause an elevation problem that didn’t exist before.
What about optically centering the scope?
Optical centering will not solve an alignment problem unless the scope really is in alignment with the bore in the first place, and it’s the internal adjustments that are throwing it off. It’s worth a look, but the odds are it will not be the solution. The best approach is to start out by optically centering the scope to eliminate this from being a cause of the problem.
Now the bad news
Sorry, but almost no scope is ever completely aligned correctly. It’s nearly impossible to do so. The target scopes use bases (and rings) that are separated by much greater length than the scopes we commonly use to minimize the effects of misalignment.
If a scope was perfectly aligned, it would be centered at 10 feet and again at 200 yards. I’ve never seen one that was. They always shift a bit as the distance changes. But they can be very close to the centerline and nobody will be the wiser. You just have to re-zero when you change from a 100-yard zero to a 200-yard zero. Or, if you shoot an airgun, when you change from a 20-yard zero to a 45-yard zero.
But the problem that started this report was a 7 centimeter (3 inches) shift to the right at 44 meters when the rifle is zeroed at 11 meters. That’s too much of a shift and can be corrected by the methods described in this report.
The good news
The good news is that a majority of airgunners will notice this phenomenon and may eventually learn how to deal with it. But the majority of firearms owners don’t even know it exists. They’re still in the world of “bad” scopes that they sell to buy name brand scopes that have to be trusted because they cost so much! In the end, you’re better off knowing the truth.