I bought a stripped Cav Arms MkII polymer lower receiver. I also bought the Armalite/Eagle parts kit for the build. The parts kit was $65.00 because it includes a carbine buffer and recoil spring. The build was easy. Took about 40 minutes. Alot of that time was spent degreasing and relubing the parts. I got way more than my fair share of packing oil with the parts kit.
As a result of obtaining this receiver I now have 14.5 (16 with comp) flat top that weighs a little over 6 lbs. with an Eotech mounted. Its fun to hand the gun to people and here them say, "Hey, this thing is light. Of course. a loaded 30 rd. magazine makes it even heavier. Another advantage is that this lower is not finicky about what upper gets put on it, unlike my RRA lower that will not take a BM upper.
An upper is mounted onto the Cav Arms polymer lower receiver using pins that have built in detents. In some uppers, the pins (with built-in detents) go right in and out. However, a little a punch help is needed on some uppers to remove the pins but they can still be pushed in by hand. None of my uppers rattle or move in this polymer receiver. All of my magazines do rattle in the oversized mag well of the Cav Arms polymer lower but that is something I do not care about. I put a Hogue Handall on the grip, which fit perfectly, to deal with my sweaty hands problem. I am at about 1100 rounds and nothing has broken, which I am usually pretty good at doing.