Here's another option for a subsonic AR-15. This 600 grain bullet will shoot subsonic from a 16" Alexander Arms Beowulf rilfe with the standard 16" twist. The bullet is available from
http://www.alliancearmament.com/thebeast.aspx
If I was going to bulld a subsonic 338 AR it would be a 338/7mmBR in a DPMS LR-308. 300 grain 338 SMKs could shoot from the magazine.. Will the 338 Specter shoot heavy low drag bullets from a AR-15's magazine? If not why not just use the 300 Whisper?