I'm not suggesting anyone should do this, but...
A 30-30 barrel I bought here would only lock up on one of my frames, a bit loosely.
The first shim I tried was one thickness of pop-can material. I've fired about 80 rounds through it, and never did replace this one with a steel one. It just feels so good that I decided to leave it there and see how long it lasted. I'm getting 1-1/2 inch groups over the hood of my Blazer at 50 yds which is plenty good enuff for what I use it for.