300 whisper or as it can be called.... 300x221 or 300 fireball and many other names. It is a super sweet subsonic round that will ususlly cycle with a can(AR-15). I know they use pistol gas systems in the AR-15 version. I think a 12" model threaded 5/8 24 on a heavy barrel would rock (threaded barrel shoulder needs to be .90" or bigger so the can will seat properly). These should be made with an 8" twist barrel so it will stabilize a 220-240 Sierra match king. 300 whisper is all about subsonic heavy bullets that sound like a movie gun. The bullet impact it the only noise!
John Noveske is a good friend with a ton of info on this setup. I'm sure Robarm will do the research before they build.