Shimming a scope

My sister in law bought my brother a Sightmark Photon for Christmas. He took it out this past weekend to sight it and possibly hunt. Make a long story short, he was 12" high at 100 yds without any elevation left. I shimmed the rear scope ring and was able to get it bore sighted at 60 yds from the house. Is there any problem with shimming the ring? Is there a better way to fix the issue?

How is it mounted?

I would check the mount/rings, then check the screws, and if it still had issues send it back to Photon. The last thing you want on a hunt is for your shim to fail and your accuracy goes to crap.

It's mounted on an flat top AR with high rings (I don't know what brand he bought), mediums wouldn't clear. Other than just throwing different brand rings at it until we found some that worked, I didn't have a different solution. I don't think its an issue with the scope. I just think its mounted too high above the bore to function properly. I haven't shot it since I bore sighted it, but I don't see any reason why it wouldn't zero. Keeping zero may be a different story.

I had warne 2 piece ultra high rings on my photon with no issues somethings out of wack . Mount the photon on another gun see if there’s any issues

Had similar trouble with a savage 243/vortex viper.

I took the 20MOA base I was using and swaped for a zero. Still didn't have enough adjustment. Swapped out the rings I was using for Buris Signature Z rings at the recommendation of someone on here and got it sighted in. The Buris Z rings have inserts in them to align the scope. They have worked great for me.

Are you sure the elevation is being adjusted the right direction?

Metal shims failing is the absolute last thing I’d ever worry about. The problem could be the rings, the receiver or the scope. The easiest possible issue to eliminate is the optic. Mount another optic on the AR and if the problem is duplicated you know it’s the Photon. Next try another mount. The receiver being out of spec is the least likely scenario.

Or if it is working then leave well enough alone which is probably what I’d do.

You need to get the scope looking at where the rifle's hitting. If it's hitting high and you don't have enough adjustment left, you need to raise the front end of the scope some---not the rear.

Something sounds all wrong to me.

Try to switch the front and back rings and see if it will zero?

It could be that the rail itself is machined poorly (unlikely), or the rings are machined poorly (also ulikely), or something going on with the scope (probably slightly more likely than the other two).

I'd swap rings around and eliminate that as a possible source. It's incredibly unlikely that the rail would be machined off-kilter. If it's not the rings, I think it's probably the scope.

Nothing wrong with shimming a scope ring or mount as long as the shim is in tight and can't fall out.

But I think it is odd that you would need to shim an AR15.

Was there another scope mounted on the AR before the Photon? If so are you using the same rings as were used on that scope?

IF you had a scope mounted previously and did not have elevation issues, then the receiver is not to blame, if you used the same rings then the rings are not to blame.

If the previous scope used different rings then the rings could be the problem. Swap the rings front to back to see if that changes things.

And as others have stated, if you are shooting too high and don't have enough adjustment to bring your hits down you should be shimming the front ring up, not the rear ring.

This did have a different, traditional scope with different rings. The old rings were too low, the scope wouldn't mount without touching the base so he bought new ones. Those too (medium), were too low for the scope to clear the base. He went with "high" rings and was able to properly mount the scope. This scope is just big and takes up a lot of room in the front, these rings are significantly higher than the ones with the original scope. When he went to sight it in, he was shooting 12" high at 100 yards without any adjustment left to lower the POI. I inserted a small shim in the rear scope ring to bring the POI down and was able to bore sight it with a laser bore sight out to 60 yds with plenty of adjustment either way. Shimming the front sounds backwards to me, seems like that would raise it even more. Just my thought process.

I had thought of switching the rings front to back and didn't do it, I will try that tonight.

Again, I do not see anything wrong with the scope to this point, I will shoot it this weekend and see how it does. If it won't zero, I will contact them and see what they say.

They'll tell you what we've already told you.

“Shimming the front sounds backwards to me, seems like that would raise it even more. Just my thought process.”

It is counter-intuitive big time to shim the front ring/base when you need to lower POI(at least to me), but that’s how it is done.

Shimming the scope changes POA not POI. The gun is going to shoot where it’s going to shoot. If the gun is shooting high and you can’t adjust higher, you raise the front. If it’s low and you can’t adjust lower, you shim the back.

The rule of thumb with iron sights is you move the rear sight in the direction you want the point of impact to move. It's the same way with the scope mount, but in this case you can't lower the rear of the scope to lower the point of impact, so you raise the front.

Look at the angles involved. When you raise the back of the scope, the front end of the scope points down but, that raises the muzzle relative to the scope making the gun shoot even higher.

If the little rectangles are the scope and the big rectangle is the gun barrel this is what happens when you move the scope.

That is a great illustration. I’m going to plagiarize the hell out of it if you don’t mind.

I don't know if its a great illustration. I'm not much of an artist, at least I don't eat the crayons.

Feel free to use it however you like.

A picture is worth a thousand words and it is very counter intuitive when you are moving sights or a scope.

Thanks.

So, here's the update. I appreciate everyone's help. I started at the range from scratch. I removed the shim and swapped the rings front to back. After this it was shooting low and not high like he said. Long story short, I was able to zero at 100 yds. The settings were -15 on the elevation and -7 on the windage. He still has some adjustment left either way. I don't know what his problem was, I had it done in less than 10 shots after I figured out how to work the scope.

Glad you got it worked out.