Very simple and rugged design. The piston is not in several pieces with extra springs and pins like the HK one. It uses a modified standard M16 carrier, a regular bolt(rings not needed) the piston is one piece and locks right in. The charging handle floats like an AR180 type, with no chance of coming out until the upper and lower are separated and the carrier is removed. The upper looks like a DPMS low pro. 16" Cav Arms chromelined lightweight barrel, (not as thin as the early Colt barrels). Predator rail system. Front sight is kind of blocky, but it functions great, the post looks like an AK47 type sight post.
I put a MAD flip-up sight on the back. I haven't shot it for accuracy yet, but all rounds were within a few inches at 100 yards. Not sure what sort of optic to go with yet. I've already put 500 rounds through it with zero malfunctions, using South African, winchester, Federal, and Russian Silver Bear. I got it hot to see if the laquered stuff would stick, but couldn't make it happen, it functioned like it should. I shot it next to my Colt sporter lightweight, the 137 looked damn near unfired, the Colt was a filthy bastard.
There was a little carbon that creeped up the piston, but it doesn't make it to the chamber, firing pin, or bolt. I tried the upper on a Rock River lower and on a Colt lower with no problems, the upper was really tight with the Colt lower. I haven't weighed it yet, but it is a little heavier than the Soprter Lightweight.
Overall, I'm very happy with my purchase. Ryan is a great guy to deal with too. Top Crest is his user name, www.hayesresearch.com for a pic of the set up. Feel free to email me with questions.