We have heard and seen this problem on more TAC15
's lately than you would believe.
The advice that came from WildWillie is the correct solution. Super 91 did 3 crossbows in the past two weeks that all had this same problem. He uses thin walled PVC as his shims and he's stated that it works very well.
I think it was WildWillie that found out that Burris makes a set of rings and a shim set that is made to take care of this problem. There are different sizes available for different scopes. It allows both vertical and horizontal adjustment, so it's perfect for solving this problem.
Russ can you give them the specifics on the rings and shims that you told me about in your lat e-mail?