SCOPE SHIFT [/b]
This is the No. 1 problem shooters have with scopes. It's also called point-of-impact (POI) shift. There are several reasons for this and none of them are the scope's fault. They're problems that plague mostly new scope users, but they can also crop up when a veteran shooter starts using a scope in a new way. The reasons for scope shift are many, so I'll try to rank them by the commonality of occurrence.
FOR SPRING GUNS ONLY: For spring guns, the No. 1 cause of scope shift is how the gun is held. Certain spring guns are so sensitive that the group impact point can be moved several inches at close range, depending on where the hands are placed and how the stock is held. The reason for this is the vibration patterns that spring guns have when they shoot. Other powerplants don't have these vibrations and cannot react to them. If you own a spring gun and don't hold it exactly the same way for every shot, you can expect poor accuracy and scope shift all the time. The solution is to develop one hold and memorize it so you always apply it.
1. Parallax
This is generally the most common cause of "scope shift" for all air rifles (with the exception of hold sensitivity for spring guns, mentioned above). I put the quotes around the phrase, because it isn't the scope that is shifting...it's the shooter! New shooters see that their scope has a parallax correction adjustment (the AO wheel) and they assume that once they're focused on target there's no more parallax. That's incorrect. Parallax is the line between your eye, the reticle and the target. Because the three are in different planes, there MUST always be some parallax. A good scope can reduce this to a large extent, which for some shooters is all it takes. Does a deer hunter care whether the bullet passed between the third and fourth ribs or just broke the third rib? As long as the deer is down, that's the whole point. But, a field target shooter gets upset when his pellet lands 0.04" from where he intended. The deer hunter will never notice parallax; the field target shooter will obsess over it. (http://www.pyramydair.com/site/articles/scope-shift-barrel-droop/ao.jpg)
The adjustable objective, or AO, removes as much parallax as possible at any given range. The shooter just turns the adjustment ring on the objective bell until the target is as sharp and clear as it can get. As good as it is, AO doesn't remove any of the parallax caused when the shooter places his aiming eye at different places in relation to the eyepiece. A repeatable spotweld that always locates the aiming eye in the same place is necessary. To correct the kind of parallax I'm talking about, the shooter has to be 100 percent certain that he places his eye at the same location for every shot. This is called a "spotweld" by the U.S. Army, which refers to assuming a position so regular that it's as if your head were spotwelded to the gun. This most-common cause for scope shift is 100 percent user-caused and can therefore be entirely prevented. 2. Elevation adjusted too high
The second most common cause for scope shift is when the vertical reticle is adjusted too high. The reticle is etched on a tube called the erector tube. When adjustments are made, the entire erector tube moves. A spring opposite the vertical adjustment knob keeps tension on the erector tube. On many scopes, this spring relaxes when the elevation is adjusted too high. I call it "floating" the erector tube, because that's what's happening...the tube is floating at the end of the spring travel. The erector tube is no longer held rigidly between the spring and the adjustment screw, and it hops around from shot to shot, taking the rifle's aimpoint with it. (http://www.pyramydair.com/site/articles/scope-shift-barrel-droop/elevation-knob.jpg) |