Every light year or so, this problem comes up............I have seen it mostly on Remingtons.
You might try swapping the rings front to back just for grins, but probably will have to shim the rear base.
I have made shims from stainless shim stock, but I think Brownell's sells shim stock for this porpose, and maybe, precut shims. Longer screws may or may not be needed, but Brownell's should have those, too.
1-800-741-0015------ask for tech support.