AR-15 Lowers

Pretty much all the forged lowers are more or less the same. In fact there’s only a few different forges that make the lowers; mostly the AR makers just buy a machined lower forging and stamp their name and serial number on it, and maybe apply a finish. One of the new trends is machined lowers. There is some argument over whether forged or machined lowers are stronger, but it probably makes very little difference, and it really comes down to aesthetics.

Who Makes My AR15 Lower Receiver? - AR15 Armory.com
List of some of the AR lower forges, and who they make lowers for.

Mega lowers are textured at the front of the magwell, to use as a grip. Spike’s and POF, to name two, make lowers that have HK-type pictograph selector markings. JP Rifles lowers have racy-looking machining. Most all other lowers are pretty much the same in terms of appearance, features, and compatibility. Almost all forged lowers will be virtually identical to an M16A2 lower, except for the maker's mark, and the lack of an auto sear pin hole and the word "Auto" or "Burst" behind the hole for the selector [although there are a few semi-auto lowers that appear to have an "auto" setting].

FWIW, Spike’s Tactical lowers look pretty cool with the spider logo and the HK selector markings. The writing, including the serial number, is deep and very clean looking. If I were going to buy another lower, a Spike’s lower would be at the top of my list. Some of the Olympic Arms lowers have nice, clean, markings in the military fashion, but the serial number seems to be stamped on later, and Olympic has been making lowers for a long long time, and I’m sure they are not all the same.

Colt lowers vary pretty widely. Old ones are slab-sided with no fence around the magazine button, like pre-A1 M-16s. Many have sear-blocks that can interfere with some aftermarket triggers. Most of the newer civilian rifle lowers require larger diameter trigger and hammer pins, and require special trigger components. The “large-pin” lowers probably have a screw-in pivot pin which will require a special conversion pivot pin to use with any non-Colt upper.