I posted this in Pistol shooting as well but didn't get any responces.

I have a S&W 647 in .17hmr. I bought a B-Square scope mount and weaver rings(Medium height) and a Nikon scope. I went to the range to sight it in and I ran out of elevation adjustment on the scope before I got it sighted in. I could only get to within 8" low. I tried to shim the scope in the back ring with a couple of layers of aluminum can shim. Went back to the range and was still about 6" low. I don't know what to try next. I would appreciate any suggestions.

Sounds to me like the problem is with the B-Square mount. Throw it away and get a decent one

Like which one would that be?