View Full Version : Scope Off Center
February 21, 2009, 16:01
I recently picked up a J. Stevens Springfield 87A semiauto .22 which was in mint shape, made between 1938-45 and identical to the one I had as a kid. The receiver was already drilled and tapped on the left side so I put a scope on it using the old Weaver side mount. The scope sets slightly to the right off the centerline of the rifle. Probably about 1/4" or so.
How does this affect the line of sight for shooting? Any issues?
No simple way to get the scope centered over the barrel.
February 21, 2009, 16:30
If I'm reading he numbers correctly, according to the calculator at
http://www.eskimo.com/~jbm/calculations/traj/traj.html the "value" of a quarter-inch sight offset (sight height guesstimated at 1.5") on a .22 rimfire rifle zeroed for 50 yards is a mere half-minute of angle.
So, in the present instance, you have to crank an extra half-minute onto your scope, over and above what might otherwise be required to establish a 50-yard zero.
Aint hardly nuthin'.
February 21, 2009, 16:39
As you can see from the chart above, sight-offset is less-and-less of an issue as the target becomes more distant.
When I first started looking at your question, I tried to think of it as an issue of rifle-cant. However, that would be the incorrect way to analyze this question, as we discover that the downrange effect of RIFLE-CANT becomes greater as the target becomes more distant.
This seems to be borne-out by plugging-in essentially the same numbers as used for the above chart, but instead using a five-degree cant, instead of the quarter-inch sight-offset.
See how the downrange effect (measured in MOA) becomes greater as the distance increases.
February 21, 2009, 17:13
If I remember correctly the instructions for zeroing a SUIT with is offset from the bore, said that you should carefully site so your group was exactly the offest the otherway from the point of aim.
The reason given, was that in that case the point of impact would be exactly the same distance from the point of aim no matter what the range.
If, for example, you adjusted so that there was no offset at 100 yds., the error would be equal to the offset at 200 yds, and 2x the offset at 800 yds etc.
This all assumes that you will be shooting at widely varing ranges. If you intend to shoot the 22 at things from between 50 and 100 yds, set the windage offset to zero at 75 yds, and don't worry about the 0.833" error at 100 yds and 50 yds. (assuming I have done the similar triangle proportion right)
February 21, 2009, 17:15
Somehow I left out a zero. In the example the error should have been 0.833" (1/12 ")
February 21, 2009, 17:19
:mad: Third time is the charm:
February 21, 2009, 22:45
I never really thought about just zeroing it in to shoot to the left by the amount the scope is to the right. That would make the bullet trajectory parallel to the line of sight and in theory I don't think the range wouldn't matter.
If I get good enought to shoot within a quarter inch at 100 yards I will be sure and let everybody know. :) Actually the thing is in really good shape and I was getting some unbelievably tight groups with some of that Wolf Match that is made in Germany.
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