Depends on the scope, the hammer is where the clearance issue is, scopes with small power rings/ocular bells are easiest to mount lower, see the scope and rings links in the FAQs and Help sticky. I'd stay away from the Weaver Grand Slams, the chinese crap they make now are a sure way to ruin a good scope unless you spend a LOT of time lapping them, they're the worst rings I've even mounted, the Leupold PRWs are excellent rings, lows can be used it you modify the hammer, that's in the FAQs too.
Tim