Mounting and Zeroing a Scope
By Terry Hart
Whenever I see folks commenting that "just getting on paper at 25 yards" with a new scope installation is adequate I shudder. If you care about accuracy then much better is possible, and without spending lots of money or time.
To make sure that I have not become completely senile in my old age I performed the following experiment. I have a Thompson Contender Rifle, or Carbine, in .221 Remington Fireball Caliber. It came from the factory with a Weaver style one piece base that has never been removed. I selected this gun for the experiment because it is generally very accurate and consistent.
First I removed the rings and scope. Then I zeroed the scope using V Blocks. The metal ones purchased many years ago are packed away in the attic so I found a cardboard box that was the right width to lay the scope across and using scissors and a ruler measured and cut a pair of V triangles into it 2" on all three sides. Then I laid the scope into this crude creation and aligned the box and cross hairs with a utility pole visible out my office window some 60 yards distant. After flipping the scope 180 degrees for both the vertical and horizontal cross hairs, and adjusting accordingly, I ended up with an approximately zeroed scope.
I have done this hundreds of times and it is amazingly accurate. New "out of the box" scopes are supposed to zeroed from the factory, and they usually are, but they should always be checked to be sure. The more distant your target the more precise the zero will be.
Next I reinstalled Millet Angle Loc rings and centered each ring above the base. Then I laid the centered scope into the rings and lined up the Windage cross hair on the utility pole. Carefully, and without moving the barrel, I then sighted through the barrel on the Utility pole. The scope was pointing 4 or 5 inches to the right of where the bore was. This is a typical error for many guns. Usually it is caused by the base being misaligned with the bore by some small angle. Rifle bores are never perfectly centered in the barrel.
I then adjusted the front ring 1/4 turn to the left and the rear ring 1/4 turn to the right. I had to repeat this process three times to get the barrel and windage cross hair both lined up with the utility pole. This means the front ring ended up three quarters of a turn to the left of center and the rear ring 3/4 of a turn to the right of center.
Sometimes the entire base will be offset from the bore to one side or the other. I have never found a simple way to tell for sure if this is the problem, or it the error results from the angular misalignment described above. It is usually safest to assume that it is caused by the later, rather than moving the entire scope right or left.
In this case the scope is mounted 1.32 inches above the bore, and as accurately as it is possible to see using this procedure, it looked about right. The only way to correct a vertical misalignment is to use base shims. In most cases this will not be necessary. With the target 50 yards or more distant If the vertical error appears to be within plus or minus 1 inch of the scope mount height, I ignore it.
Finally I installed and tightened the ring tops, or caps, and checked the alignment again. This entire procedure takes less than an hour and requires no special equipment.
Note that I left the ring tops, or caps, off while I was doing these adjustments. If the holes through the two rings are excessively misaligned the scope will try to crawl up out of the ring. Anytime the rings are offset from each other it is a good idea to use a 1 inch piece of wooden dowel wrapped in emery paper and to lap the rings so the holes are exactly aligned with each other. I did not do that in this case and it doesn't seem to be causing any problem.
Today I headed for the range to check the results. The temperature was 29 degrees with the wind blowing from directly behind and towards the target 15 to 20 miles per hour.
With any new scope installation I always start at 25 yards. The only ammunition I had available were Nosler factory loads with 40 grain boat tail bullets. This stuff screams out of the muzzle at something between 3,200 and 3,300 feet per second. My barrel has a 1:12 twist and doesn't especially like this load, but it was what I had. The results are as follows:
.442" Right and .656" Low
Average .1344" Right and .6522" Low
Overall spread .440", and if I throw out the first cold clean barrel round as a flier, .387". With 50 or 55 grain bullets backed off form maximum load a bit this gun typically yields smaller one hole groups.
The point of this experiment was to show that it is relatively simple and totally painless to mount your scope so that you don't need to screw it half way, or more, to the adjustment limits in order to hit what you aim at. My rule has always been that the group should be within plus or minus 1 inch at 25 yards before beginning to screw on the scope. If it isn't the mount needs more tinkering.
Copyright 2005 by Terry Hart. All rights reserved.