AR Build, Lower Receiver Group

If you are new to the AR platform, the rifle breaks down for cleaning into two main groups, the upper receiver and the lower receiver. To the left you can see my lower receiver group with the various parts marked.

Technically, the lower receiver "is" the weapon. Even though the upper receiver includes the bolt, chamber, and barrel, the lower contains the trigger mechanism, and as such is the part of the weapon that the United States has determined constitutes the weapon. As a result, any lower receivers purchased in the United States must go through an FFL dealer. My lower is the one that came with my Bushmaster, but you can buy just the lower if you want to build "from scratch".

Lower Receiver Group

 Let's deal with the actual lower receiver group (LRG) and trigger mechanism first. Probably any LRG will work for your build, but if you are working from scratch, I highly recommend using a reputable company, specifically one that supplies to the armed forces. BCM produces an excellent lower receiver group.

A word on Mil-spec. "Mil-spec" or "MIL STD" refers to "military specification", and means that the parts have been manufactured according to rigorous tolerances and subjected to treatment processes that ensure a very durable weapon. Durability is one of the most important aspects of building a wilderness weapon - you want one that you can fire, drop in the mud, fling over a canyon, fire some more, and then do it again tomorrow. However, true mil-spec lowers are not available for commercial sale in the United States, as they include a full-auto trigger mechanism, which is not legal for sale to civilians. There are three main choices when it comes to LRGs; first, the unit can be cast or machined from billet; second, there are various finishing choices; third, if you purchase one with the buffer tube already installed you have the option of mil-spec or commercial-spec tubes. We will talk about the buffer tube when we discuss the buttstock. For now, let's focus on the other two.

There is little practical difference between a cast lower and a billet lower; the real difference is in the manufacturing process. While billet is generally stronger, you must find out what alloy the receiver was machined from. The process is irrelevant if an inferior metal is used. At the same time, the manufacturer must use high quality processes or the difference again becomes irrelevant. Your best bet is to go with a reputable lower; leave the technical specs to the engineers. Again, any commercially available AR lower from mid-grade to high-end rifle manufacturers will suffice, but if you are building from scratch, I recommend a lower machined from 7075-T6 forgings.

The finish is an important aspect of the lower, as it will protect the weapon from oxidation and decay. Aluminum doesn't rust very easily, but it does rust. I recommend a hard-coat anodizing process, specifically the MIL-A-8625F Type 3, Class 2 coating demanded by the U.S. military. Some receivers come with a Teflon or similar coating. I am hesitant to recommend Teflon for two reasons. First, it can scratch off. Anodizing actually permeates the first few microns of the surface, rather than being a coating. It is magnetically "sucked in" to the metal. Teflon, on the other hand, can scratch off. Also, Teflon adds a very, very small thickness to the part, which can affect tolerances. That being said, I have never heard of anyone complaining on either account. In the end, the difference may be academic, but the first time you drop your weapon into a ravine and it takes you two days to recover, you might be thankful for all the durability you can get.

Get yourself a sturdy lower from a reputable company, and get building.