The lower receiver for an AR 15 is most commonly made from anodized aluminum. Aluminum makes sense because they are lightweight, the material is abundant and affordable. However, a receiver doesn’t necessarily have to be made of aluminum. In many cases, a receiver can me made from a variety of materials. There isn’t necessarily a lot of pressure on the lower receiver when the weapon is being fired.
In fact, there is almost no stress on the lower receiver. With this in mind, a receiver can be made from polymer. Polymer receivers are a new invention. Only a few have come into the mainstream in the last few years. The idea of using polymer seems viable in some considerations. A polymer lower isn’t a risk of exploding because it’s polymer. They can withstand the pressure from the meager 5.56 round easily enough.
Advantages of polymer A few advantages come to be pretty quickly once you consider polymer over metal. If the current trend of pistols is any example then polymer lower receivers could be a very real possibility in the future.
The first advantage is of course weight savings. AR 15s have been trending lighter and lighter for years. The use of keymod and Mlok rail systems have been a big leap forward in lightweight modularity. There are even carbon fiber barrels, although they are expensive and somewhat rare. A polymer lower receiver weighs about a full pound and a half lighter than an aluminum receiver. That’s a considerable amount of weight to shave by just swapping receivers.
The next advantage comes in one of the truest advantages of polymer over metal, corrosion resistance. Polymer doesn’t rust. The finish is built into the receiver. If the receiver gets scratched or gouged it doesn’t result in exposed aluminum. The surface and what's under it are the exact same thing.
Polymer receiver are also much easier to produce. Easier to produce receivers generally means cheaper receivers. The average price of the cheapest metal receiver I can find is 49.99. The cheapest polymer lower I can find is 35 dollars.
The last advantage is not an important one. However, like all polymer, it can be easily modified to create unique designs. You could theoretically stipple it, dremel designs into it, or just about anything else you’d choose to do. Polymer is also pretty easy to paint and in general, can be customized extremely easy.
Disadvantages of Polymer So everything new is bound to have some kind of bugs involved with it. Some times new technology works , other times there can be serious issues. Even the most reputable companies have problems. For example, Glocks are legendary for their reliability but who can forget the generation 4 issues? Polymer lower receivers are no different they have certainly had some teething issues.
The ATI Omni lower receivers are one example of issues. One of the biggest is a durability issue. The area around the buffer tube is often the thinnest area of a receiver. Yet, it has to take the most stress. A big problem with some of the first poly lowers was the area around the buffer tube snapping. This ruins a receiver completely and totally.
Some enterprising companies, including ATI and MEAN, are producing hybrid lowers with metal reinforcement. These seem to eliminate the buffer tube issue. The next problem with polymer lower receivers is the flex factor. Not present in all poly lowers, but in enough lowers to create a real problem. The flex factor is when parts of the rifle flex under recoil. This can cause serious malfunctions, including failures to feed and fire.
There has also been an issue with receivers being out of spec. Aluminum receivers have been used for so long making one is fairly simple. Polymer lowers are new and seem to have issues staying in spec. It doesn't seem to be as simple as a standard aluminum receiver. These out of spec receivers may not fit certain parts properly, and cannot be used in a build.
I’ve also seen issues using certain additional accessories with polymer lowers. Some ambidextrous safeties will not function with poly lowers. In other cases, accessories like Magpul's bad lever may not fit correctly.
In my personal experience, I’ve purchased a poly lower, installed almost all of a lower parts kit, and then hit a snag. The first one involved the rear take down pin. The detent hole was not wide enough to allow the detent to actually engage and retain the rear take down pin. The next problem was that the lower could not mate with an upper. They were completely out of sync with each other.
That's not to say the polymer lower receiver is a bad idea. However, I am cautious about purchasing one without doing an excessive amount of research. The idea and concept of polymer lowers is valid. If modern handguns haven’t taught us that polymer is the future I don’t know what will. For now, there are growing issues we need to address. If you want to go polymer consider their faults and do your research.
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