Thanks, Spook. I found this on the 500 Phantom.
http://www.feistyrooster.com/500phantom/index2.html
I appears to be exactly what I am looking for. Suppressed, short stroke piston AR10 type rifle (LWRC). I have several other subsonic projects ongoing at this time but perhaps brass will be more readily available when I am ready to take the plunge (probably a year or so). OAL might make a bolt action more realistic, though.
David