One topic everyone discusses to death is quality of AR lowers, while I would give an ordered list of quality lowers, there are just too damn many of them. But here are a few pro-tips from someone in the industry (myself). 1) Mil-Spec: There really are no mil-spec AR-15 lowers, because the military doesn't buy semi-auto AR's, none of them will be made to mil spec, so if someone says a part is "mil spec" you should be able to look up and cross reference which specs it complies with. Examples: If the anodizing is Type 3 hard-coat sulfuric acid anodizing, then it could be compliant with MIL-A-8625. Steel parts of the gun typically conform to MIL-STD-171 which covers manganese and zinc phosphate coatings on steel parts for corrosion resistance. The parts of the AR-15 that will never be mil-spec without an NFA tax stamp: Lower Receiver, Bolt-Carrier, and Barrel (must be 16"). If you are producing something to look like an M4. In general, nearly every aluminum lower starts life as a drop-forged chunk of aluminum made by 3-4 companies in the US, all of them are made out of 7075 aircraft aluminum and are heat treated to a T6 or better hardness. Plastic lowers and cast lowers usually do not conform to the normal material specs either because they're plastic (no mil-spec here) or because 7075 is a poor alloy for casting. Billet lowers may be made out of 7075, but there really is no requirement to do so in this case, as anyone with a CNC shop can start making billet lowers out of any of the commonly available aluminum alloys. (6061 T6 is a much cheaper, and still fairly strong alloy, but it is nowhere near as tough as 7075) 2) Fit and finish - One thing I have been seeing a lot more of lately is very poor fit and finish on AR lowers put out by a number of companies, the principle offenders I've seen are Colt, Del-Ton, Aero Machine among many others. If you are thinking about buying one of these lowers (or the complete gun) run your fingers along the edges, especially along the back strap just above the hand grip and inside the trigger guard, if you feel a thick irregular mold line, make sure you're not paying a premium price for this lower/gun. What this means is in the rush to produce the lower, they couldn't be bothered to run an end-mill along the edge of the lower and cut that mold line off, this line is left from when the lower was hot-forged where the mold lines met, and extra aluminum was expelled. If you look at an S&W M&P15, they do a very good job of making sure this mold line is removed from the lower. Just for reference, some manufacturers do different things with different models, some of these are good, some are bad, some are indifferent. Getting back to the mil-spec point. Some of the newer "billet" lowers have dropped things like having the trigger guard as a separate piece, so forget about putting your own after-market part on there. The S&W M&P15-sport is a key example of this. However, the normal M&P15 is as close as you are going to get to an M4 without NFA taxes, it's a very high quality product. I'll try to keep this list up to date.