Generally, to mount a scope directly to an AR's M1913 rail without some kind of riser-type base, you (again, generally) need a minimum of "high" rings. Usually, ring companies make AR rings in "Extra High" configurations. This ensures the scope is mounted at a height consistent with the average shooter's cheek weld. There's no drop to an AR stock, so you have to get the scope up high. This is beneficial, as it clears the scope from interfering with the charging handle. Though, I've never had that problem.
I favor:
http://badgerordnance.com/30mm-scope-rings/The AR-15 extra high rings in alloy are reasonably light and as sturdy as you'll find. If you run a 1" scope, you'll have to get reducers, which I've had no problems running. Badger stuff is relatively expensive, but I've run them professionally and personally for over a decade with no problems at all.
