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Gun Talk: Scope Mounting

A Crucial Procedure

While it may seem straight forward, mounting the scope to the rifle is one of the most crucial procedures to ensure success.  Many rifles today are produced off of CNC machines and one would think that all the actions would have the same dimensions. This would mean mounting a scope would simply be screwing on the bases and rings and off to the range, this however would be wrong.  While the actions may be machined on the CNC mills, they are hand finished by employees who sand or polish off the production/tooling marks. This finishing brings in variables in bridge height. It doesn’t take much polishing to really have an adverse effect; the good news is that it really isn’t too hard to correct any issues either.

Just last week I encountered this when I headed to the range to sight in a rifle I just had re-barreled. I had used the rifle for several years but had a new barrel installed and the rifle re-blued with a nice satin finish. The rifle was polished for the re-blue and there was enough metal removed that it caused some issues. With the scope optically centered, the point of impact was well over 20” low at 100 yards on my first shot. Most people would have simply just started cranking the elevation dial to make the correction. That, in my opinion, is too much adjustment to use the dials. A scope’s optimum performance is usually attained when it is as close to its optical center as possible. I returned home with the rifle, installed a shim of roughly .024” and headed back the next day. The next shot through the rifle was within a few inches of my sighting point. I made the elevation correction with the dial, adjusted the rear windage screws on the bases to get the point of impact close and final fine adjustments with the dials. All tolled, I doubt that I used 12 clicks of adjustment in the scope. If I had simply just cranked it in that first day at the range I would have used over 80 clicks. Many people don’t realize that the two larger screws on the rear base of the Leupold STD ring are actually there for coarse windage adjustment, not just to hold the rear ring.

You have to remember that a scope is basically comprised of two tubes inside, the more adjustment you use in one direction you take from the other. When the system in the scope gets pushed over, close to the inside edge, it can no longer track in a straight line, it will instead follow the curve of the tube which is  not the scope’s fault. I have people tell me all the time that their scope didn’t have enough adjustment in it to sight the rifle in. The scope is most likely fine. Something is crooked on the rifle and mounts. Whether it is bridge heights or even the holes drilled and tapped slightly off center, the tolerances of being off center are magnified drastically as you go down range.

A shim of .012 under the rear base will give you about 12 inches at 100 yards, give or take. Shims go under the bases, not in the rings. Naturally there are certain platforms that you cannot shim or use the windage screws in the bases, so there you must use scope adjustment to correct the differences in tolerances. This is the same principal long range shooters use by “adding a 20 MOA” base to their set up, it allows them to be able to extend their elevation range of their turret/dials.

We need to get passed the idea that shimming is wrong.   Unless one has access to a mill, to make the height adjustments in the base themselves, it is the only correct way to take up the excessive tolerances. I have overseen and mounted countless scopes in my career and the use of shims is required more times than not. Hard to believe, but true; we live in a world of tolerances
not absolutes.

So as we prepare for our upcoming hunts and head out to the range, give yourself enough time to work through any problems that you may encounter. As tedious and frustrating as a crooked action is, it is not hard to diagnose or to  then correct.

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Matt Siemens

Contributing Author