So don't use it? Odds are pretty good that if you spawned with the AR you also spawned with either the pistol or the DMR, which are both ten times more useful.
One thing I don't miss about Halo 3 is getting in a Slayer game and watching people do this over and over again:
-AR AR AR AR AR
-TRADE MELEES, MUTUAL DEATH
It's also terrible in H3 if they have a BR and are at an appropriate range, they're just going to four shot you while backing up.