Scout Scope Mounting Problems

I recently mounted a Burris 3-12X pistol scope on the built-in rail on my M1A Scout Squad, but have been having problems with it shooting something like a foot high at 25 yards! I've put a couple of brass shims under the scope on the front ring, so I can zero it, but the scope is still run up to almost its maximum elevation.

I tried reversing the rings, thinking one might be slightly shorter, without any change. Since the Burris pistol scopes are the same as their long eye-relief scout scopes, I wouldn't think the scope was off spec.

Has anyone else had this problem with the scout mount? Could it be that I got one that's out of spec?

Would appreciate some sage advice. I've mounted scopes before, but never a scout scope.

Thanks