ANOTHER problem with scopes: Not mounting them correctly
by B.B. Pelletier
This one comes up a lot, so I thought I'd mention it in case any of you are experiencing it: Your gun shoots to one side at close range and to the opposite side at distance. I'm going to explain the problems and how to deal with them, then I'll give you all the references to past postings where I've explained other pertinent things.
The scope is not aligned right
There are two things that will cause this problem, as far as I know. One is if the vertical reticle is not truly vertical but actually on a slight slant. At close distances, defined as those distances before the pellet first intersects the crosshairs, the scope shoots off to one side. For me, it's always off to the right - the closer I shoot, the more it's off. So, at 10 yards it will be off half an inch to the right; at 17 yards it's off a pellet diameter and still to the right.
At far distances, which are those beyond the second point of pellet/crosshair intersection, the pellet strays to the other side of the vertical reticle and keeps diverging as the distance increases.
My sight-in distance is always 20 to 30 or 35 yards, depending on the velocity of the gun I'm shooting. If it's above 850 f.p.s., the far distance is 35 yards. Between those distances (20 and 35 yards), the pellet always lands very close to where the crosshairs are placed. Within that range of distances, the pellet seems to be centered vertically.
You align the crosshairs when you mount the scope
Everyone sees things a little bit differently, and this is one time when that IS NOT okay! The scope/gun relationship depends on the precision of alignment, and the sideways straying of the pellet is an indicator of poor alignment. I get it wrong all the time, so I have a lot of experience with it.
When you mount a scope, you turn it until the vertical crosshair appears to bisect the receiver of your gun. If this alignment is not true, you'll get this pellet shift problem. There is no mechanical tool to help with this situation, but with patience and understanding you'll overcome it. The scope collimator that some shooters use to assist in scope-mounting is a help, but you can't rely on it 100 percent of the time. When I notice one of my guns has this problem, I loosen the rings and turn the scope slightly to the left until it starts to look off-center to me. That turns out to be the spot at where it's correctly aligned. You're going to have to gain this same experience for yourself.
That's why I prefer to mount my own scope. If someone else mounts a scope for me, I may end up suffering with their problems instead of my own. I take scope mounting very seriously.
The other alignment problem
If your scope isn't optically centered when you sight in, you will be using some internal correction to move the pellet strike. I optically center the reticle, then I use B-Square AA adjustable scope mounts and rings to mount the scope on the rifle. I've tested most other adjustables, and they just don't cut the mustard. AA mounts are great! I sight-in using their adjustments and as little scope knob movement as I can get away with. There may be a better way to do it, but this is what works for me!
Thanks to bigvic's request, we now have an index of all past postings, which lets us see the scope-related stuff at a glance. Read What causes scope shift to get more on the scope alignment operation. Also read Another cause of scope shift: over-adjusted scope knobs to learn about another technical problem. Another great posting was At what range should you zero your scope? Although this subject is full of controversy, at least read the posting to see what is involved when you zero. Take a look at More about sighting-in: How to determine the two intersection points. It's a companion to the first one. You may be interested in How to optically center a scope.
In August, I did two posts on mounts. See Scope mount basics - part one and Scope mount basics - part two.
Wow! It looks like I've written a small book on scopes and scope mounting.
Besides my postings, Tom Gaylord has written a number of articles about scopes. You might find them helpful, too:
All about scopes, Part 1
All about scopes, Part 2
All about scopes, Part 3
What does AO mean?
What is a mil-dot scope?
I hope this summary helps you with your scopes!