I have a new bushmaster m-4 law enforcement rifle.  I attempted to install the ras rail and found i cannot get the rear clamp to clamp under the barrel nut.  It appears the rail has thicker lips and won't slide between the barrel nut and slip ring.  Has anyone had a problem with this? I wound up gauging the metal on the slip ring and the rail itself.  Is it usually this hard to install it?

It can be a painn.  Its a tight fit.  The biggest problem is getting the locking tab down and under the barrel nut while muscling the whole thing past the delta ring.

Are you trying to push down the back ring by hand? If so try one of those handguard changong tools. They only cost about $18 and they really take some of the grief out of the job. An extra pair of hands comes in handy too.