It's old (not sure what the OP's final fix) but found this FYSA ...
wrote 4 years 44 weeks ago
I bought a cz550 in 416 Rigby last year. The action is perfect. The trigger is amazing. It shot dead on with the iron sights. The only problem I had was that when I mounted the scope, it shot way too low. I adjusted the scope as far as possible, still not enough.I contacted the company, and was told that they never really intended for the big bores to be used with a scope (why the mounts then????), but they offered to replace it no questions asked. I hated to do that because I had already had the action glass bedded. I honestly think that the problem is with the rings (also cz, but they don't measure the same height), not the action. I'm going to try a different set of rings and see if that fixes it, but for now, I just put a .020 shim under the back ring. Works great. It will shoot under 3/4" at 100 yards with both Norma and Hornady ammo. As a matter of fact, I used it to shoot a five point bull elk last year. Poor bugger never knew what hit him.
Every time I've seen someone try to correct an elevation problem using a shim underneath the scope tube, the results were bad for the scope. The shim reduces the contact are between the ring saddle and scope tube. Tightening the ring screws puts dents in the scope tube. Because the contact area is low, the scope slips in the rings when the gun is fired, causing gouges in the tube.
Shims alone are not a good approach for a magnum rifle. Bedding the scope tube with a spacer in the bedding material works just fine.