Why does my rifle shift its aim point? - Part 3
by B.B. Pelletier
Okay, today I'd like to finish this report. There's a lot to cover, but I won't go into detail like I did in the first two parts. Remember, the complaint was a point-of-impact shift from one day to another AND a shift in the middle of a shot string. Those two separate reasons could each cause a POI shift, but if BOTH of them are happening, it can only be caused by parallax or a broken scope or loose mount. I think Hegshen is careful enough to catch the bad scope and loose mount, so I had to go with parallax. But there are other causes of impact shift.
Cause 1. Positioning of the cocking knob
This is an AirForce quirk, because they have a cocking knob that must be rotated into one of two locking notches on either side of the receiver. Fail to do it, and the bolt will move during firing, causing drastic changes in the amount of compressed air that gets behind the pellet. That will cause inaccuracy and POI shifts. When I was at AirForce and answered phone calls about accuracy problems, this was one of the main causes of those problems. AirForce made a video about how to operate the rifle and put the instructions in the owner's manual. If a shooter didn't pay attention to those instructions, they often had accuracy problems. Some guys went to the extreme of looping a rubber band around the bolt handle and the gun, so the handle would always be pulled into the notch when not being used to cock and load the gun.
Cause 2. Elevation adjusted too high
When the scope's internal elevation is adjusted up beyond a certain point, the erector tube return spring relaxes to the point that the tube can move at will. This will cause a wandering POI or sometimes a jump from one POI to another. This is a universal problem with scopes and the principal reason I recommend B-Square adjustable mounts. You can usually feel when the spring tension relaxes, because the clicks become softer and mushier. Avoid this region of adjustment if you want accuracy and a consistent POI.
Cause 3. Temperature change
A large change in outside temperature will change how the lenses in your scope are aligned. This is usually a cause for a POI shift when more than a day lapses between shooting, but I once saw it happen in the middle of a field target match, when a thunderstorm dropped the temperature by more than 20 degrees in a short time. The only solution is to re-zero the scope.
Cause 4. Shooting at different ranges
I tried to make this point in Part 2, but if that wasn't positive enough, I'm saying it again. Shooting at different ranges affects the zero of your scope. I'm not talking about elevation. Everyone knows pellets don't fly straight and will print at different places depending on the range. But if the scope is not aligned with the bore of the airgun horizontally, it will shoot to one side close up and to the other side far away. You will never see this after zeroing, because you will have aligned the scope and pellet flight path at one specific range. But, change ranges, and the POI will shift to the side. When the scope is optically aligned with the bore, this won't happen.
That being said, this one isn't a big problem for most scope users. When you miss your POI on a deer, you're still within the kill zone and no one is the wiser. It's only when you're looking for half-pellet diameter accuracy and get slapped in the face with a one-inch shift that you pay attention.
Cause 5. Cant
Cant means tilting the scope away from the plane at which it was sighted-in. This will throw the pellet to the side and also down. I did a cant test that demonstrated a pellet movement of as much as 6 inches at 50 yards when cant is involved. For that test, the cant was a measured 20 degrees in each direction, something that would never happen in the real world. But a 3-degree cant is possible for some people on some terrain that doesn't give good cues as to level. That can throw you off by an inch at 50 yards.
Cause 6. Sidewind
A sidewind will blow your pellet in the direction the wind is blowing, and also either up of down, depending on the wind direction and the direction of spin imparted by your barrel. This is caused by a phenomenon known as precession. A gyroscope (your spinning pellet) that's pushed in a certain direction will move at 90 degrees to the force. The movement will always be in the direction of spin. If the gyro is spinning in a righthand direction and you push it to the right, it doesn't go right...it goes down. Push from the left and the same gyro doesn't go left...it goes up. The gyroscope is touching the earth at its point where a lot of friction holds it fast. But a pellet flying through the air has no friction holding it in place, so it tends to go with the flow. Therefore, the sideways movement is usually larger than the upward or downward movement.
Cause 7. Changing the scope's power
When the power is increased or decreased on some variable scopes, the point of impact can change. Apparently, it depends on how the lenses are arranged inside the scope. This used to be a common problem, but a lot of scope manufacturers have designed their scopes so it doesn't happen as much anymore. I shoot a lot of Leapers scopes, and I've never seen a POI change when the power was adjusted on one of them.
Those are all the causes for POI shift I can think of. If you know of one I've overlooked, please respond.