The lower is just for receiving the mag, feeding the cartridges and activating the hammer with the trigger. There are only two sizes of lowers the AR10 and AR15 sizes. Any forged milspec lower is fine as far as that goes. Some guys even like some of the polymer lowers receivers. There really isn't much to putting an AR together.
Any lower will work with the 57 upper. The lower on the AR57 does nothing more than basically holding the upper in place and providing a trigger. The magazine well in the lower does nothing more than channel the spent brass straight down.
I have one of the plastic New Frontier lowers on mine. I cut the top off of an old beat up AR15 magazine to put in the mag well to catch the brass. Even that old beat up magazine is worth a minimum of $50 now.
Look at it this way - a forged lower is a forged lower. Doesn't really matter who makes it. Metal is metal. I used to work in an aluminum foundry and we made lots of different alloys.
Milling of the lower these days is done with a CNC machining machine. We had those too at the foundry since part of the factory did extrusion and we machined parts.
A CNC machine has interchangeable drills, grinders, saw blades and a host of other tools in it. With the programming it automatically changes tool heads to what is needed during the machining process. Milspec is milspec and the lower receiver is machined to the specification with tolerances of generally 1-5 thousandths of an inch, (machining in CNC machines is measured in thousandths of millimeters - i.e. micrometers). Even upper receivers are machined this way. The upper receiver has a wider tolerance level than a lower receiver.
Following machining, the metal part (for a gun) will be anodized. This involves sticking it in a chemical bath with electrical current running through it. Anodizing does two basic things: 1) it hardens aluminum. 2) it puts a finish on the metal. The type and color of the finish depends on which chemicals are added to the basic bath. Part of the plant where I managed the forklift operations to move all the metal around the plant was the anodizing facility.
So, if you get a milspec stripped lower receiver that has the machining completed, it won't matter what company you get it from. Effectively they are all the same. The main questions are: Is the price reasonable and in the correct market range? Do they have it in stock NOW so they can ship it to you?
Is AR five seven any better than any of the other companies? I doubt it. I have read reviews from numerous companies. Some customers seem always to get bad service from every one of them. Some customers send in nice reviews because their order went smoothly. I ordered a lower receiver from Alexander Arms listed as in stock, then I got a run around and it never shipped. Reviews on the company were fine.
I don't put much value on reviews anymore. Sure, I do look at them but they aren't all that reliable.
From what I see, AR five seven has only recently started making lower receivers so, there isn't much out there about them yet. Uppers they've been putting together for a while so there are reviews on that.
WHSmith, that sounds too familiar Where I work at, we have started to make AR lowers (well, let's just say 80% of it) I don't work on anything that deals with the CNC stuff though, I'm just one of the young clean-up guys there... but from what my supervisor was saying, all the major manufacturers use the exact same programs, tooling, and even vices. EVERYTHING to where you have such minimal tolerances there would be no reason to pay extra for a bushmaster than say... some brand you never heard of. In the programs, it's even as precise as "1/8" bit @ 2500 RPM, then 5000 RPM on extraction" and all the companies do it the same.