As long as the scope and float rail are both mounted properly, you only need enough clearance so they don't touch. 1/10th inch is fine. Note that on an AR-15, you need rings with approx 1.4-1.6" rail to scope center height for proper mount height. When the scope is rigidly mounted the rifle moves as a whole unit.
Last edited by Zak Smith; 08-27-2010 at 13:09..