Another option just to try and figure out where the problem is, is to try that scope on a gun you know has rings on it that work, and also try another scope on this gun/ring setup. Of course you want to make sure the current gun/rings aren't going to ruin a scope you put in them. The front needs to come up and/or back needs to go down. So you've turned the scope elevation turret as far in the "down" direction as you can go right? I agree with not liking the idea of shimming and home machining of base parts is probably not the best idea unless you do that kind of work. If this was a setup that needed to shoot "minute of deer" with max 100yd shots that's one thing, but for a rifle that has to hold zero through hikes and travel, you probably want to have everything guaranteed solid. As others have said, lapping is necessary if any significant amount of shimming is done.