Never but NEVER use shims to breech up. If your rifle needs a, say, .010" shim to get it to breech up correctly, then machine the breeching-up face back .10" and use a .110" breeching-up spacer. L1A1 breeching up spacers are a good source of such things. Shims are not made for this purpose and to do so is p---poor engineering practice in any case.

That's the difference between time served Armourers and enthusiastic amateurs