arch-iceman, what about a Colt M4 upper for your Bushmaster preban? You can still find the Colt M4 upper at Shot Gun News for $699(?). I have a Colt M4 upper and it works perfectly with my Bushmaster preban.
All standard 16" and 20" uppers from Bushmaster, except the V-Match, work with the Knights RIS/RAS. However, Knights do have a free floated RAS for their SR25, but they don't sell the free floated RAS individually.
Yes my Bushmaster Dissipator is a 16" configuration (only version from Bushmaster). http://members.aol.com:/himnic/ar/dissipator
-Preban Bushmaster Lower
-Bushmaster Dissipator Upper w/ Knights M5 RAS
-Knights Surefire Mount w/ Surefire Tactical Light
-GG&G Scout Rail w/ GG&G M.A.D Flip-Up Sight
-ARMS Scope Rings (High)
-Leupold VARI XIII Tactical Scope
-ARMS Forward Pistol Grip