i recently read that if you adjust magnification on a scope, that the zero will shift. is this true? if so, does that happen EVERY time you increase/decrease magnification?

Are you saying that after you have the rifle sighted in on say 4X if you increase to 7X the POI will change? Not on my scopes. The focus might need some adjustment but it should hit at the same spot. I move my powers around all the time. If I had a scope that was doing that, it would be time to get a new scope...

That only happens if the scope is defective.

This can also be caused by the rear scope ring putting uneaven pressure on the tube. This is one very good reason to lap your scope rings. A friend had an atec that did this and a lap job cured it.

What is a lap job?

This ihappensif the optical centers of the lenses are not exactly in the physical center plane of the lens as it is mounted inside the scope. What happens is that when the lenses that rotate turn about their physical axis, the optical center rotates off center, shifting the transmitted image in relation to the reticle.

To prevent this, one of two approaches is required-either the lenses have to be so perfect thattheir optical centers are exactly in the physical center of the lens mounts, or the power-change lens system has to be engineered so that it does not rotate as it moves back and forth inside the scope tube when you rotate the power change ring.

One quick way to determine if a given scope has such a problem or not is to insert a bore sighting collimator into therifle bore, then observe to see if the scope reticle appears to move when you turn thepower shifting ring. If the reticle does not appear to move, your scope is O.K. Of course, this test requires that the scope be mounted on a rifle......

Lapping scope rings involves putting a hardened steel rod 1" in diameter in the scope rings. You then add lapping compound (abrasive paste) and work it back and forth and in a spiral motion. This makes the rings line up and prevents flex in the scope tube which can cause all kinds of problems.

Is there any other way to prevent scope tube flex besides lapping?

Yes. Burris has a scope mount that has oversized rings with a plastic insert of some sort that automatically compensates for off-center conditionswhen you tighten the rings. I kinda forget what they call this gadget..... But it works!

eldequello, this burris ring you are referring to is the Burris Signature rings. It does help if the rings aren't lined up and also if you run out of windage on your horizontal or vertical as you can get an offset set that will raise or lower either one or both ends of the scope to avoid having to shim the base. I got it from Midway for I think $30. I put it on my slug gun and it has worked so far as the scope hasn't shifted even with magnum slugs.