I presume you are speaking of the AR10 type rifles. If so, the 302 whisper will give you what you are looking for if you are set on 30 caliber. If willing to go up in caliber, the 338 whisper on the 7mm BenchRest case has a good following as well.
The 338 was recently discussed here:
http://www.quarterbore.net/forums/showthread.php?t=4374