Been playing with this as my newest toy;
Foxtrot Mike FM-15 11.5" Buffer-less upper.
Very unique, like an unholy offspring of a BRN180 and a normal AR. It is not Piston, still Direct impingement but side charging and buffer-less. I picked up the new generation with the 3 position Adjustable Gas block.
For the total price of $530 plus Foxtrot Mike includes the rear 1913 buffer tube adapter in the kit. A killer price given other Mfg. versions of a buffer-less upper is in the 800-1000 range.
Haven't played with it enough yet to have a verdict.