I acquired a CMMG conversion set awhile back.
4.5” barrel, Bolt and collar for the barrel. Assembled into a standard upper with a 4” carbon fiber handguard. The threaded end of the barrel just stick past the handguard. I put a RDS on it and picked up some Black Dog magazines and a drum. It sits on my SBR lower and run great with the Spectre 2 on it. It will dump a mag as quickly as I can pull the trigger. The drum rocks with non-hollow point bullets. I haven’t fired for groups as it was intended as a blaster. I will do so sometime this winter.
I may eventually set it up on a dedicated lower.
I already had the upper so the only real cost was the conversion kit, the handguard I caught on a clearance sale for $40, and then the magazines.
I need to find a longer barrel and collar so I have other options for range day.