Building an AR15: Metal v. Polymer Lower Receivers
If you want to build an AR 15, your first decision has to be the lower receiver. Which is the better AR receiver, polymer or metal?
The lower is the part that houses the trigger and hammer. It is also where the upper receiver attaches and the buffer tube assembly screws in. The upper connects at two points, the takedowns. We sell both and all the parts you need to complete the build.
Both lowers are good, provided you understand the performance levels of each one.
So, which is better? Metal or polymer. Let's take a look at the deciding factors.
The great majority of shooters are not concerned about weight. They do not carry the rifle through rough terrain for hours on end. Walking to a stand, either in the woods or the range, is not a long time to carry a rifle.
Besides, the weight difference between the two is negligible. If you need to drop weight, a better way to do this is a shorter barrel, fewer accessories or even skeletonize some of the parts.
A slight advantage goes to the polymer.
Buffer tubes are flimsy, thin aluminum. The threads are easy to strip or cross-thread. You are not likely to strip the tube threads in a polymer lower unless the lower has a reinforced mounting ring. If the lower is all polymer, you are more likely to strip or cross-thread the lower receiver.
If you strip the threads on a tube, you are out a few bucks for a new one. Strip the threads in the lower receiver and a repair costs more than replacing the lower if it can be repaired.
With a reinforced tube connection, this is a draw.
The weakest point on an AR lower is the elbow at the buffer tube. Polymers are notorious for breaking at this point.
Some companies came out with a setup to cast a polymer lower at home that makes this point very clear. Note: These are not legal in Delaware. We discuss them here to make a point. Broken lowers at the elbow made these companies start including steel wires to insert into the casting. The wire reinforces this weak point. The first-generation home-cast polymers sometimes did not last for a full 30-round mag before showing cracks.
Even so, home-cast polymer lowers do not hold up to heavy recoil. They are not tough enough. A .450 Bushmaster can destroy a polymer lower pretty quickly. Metal will stand up to the punishment.
The advantage clearly goes to metal.
If money is a consideration, go polymer. They cost less than metal lowers. Some high-end metal lowers go for $200 or more. (Most metal lowers we recommend are less than $200)
The advantage is to the polymer being the cheapest option, but the old saying “you get what you pay for” holds true.
The trigger system and hammer have two major springs. These are the internals. The hammer spring is by far the stronger of the two. You can buy a kit and assemble the internals or buy a pre-built drop-in kit. Both kinds of high-end kits have adjustable trigger pulls.
Polymer lowers do not have the strength to hold up to heavy hammer springs. Gun journalist Ben Baker was trying to install an Anderson lower parts kit in a polymer when the retaining pin ripped through the sidewall of the lower. The hammer spring put too much pressure on the sidewall.
These pressures are the main reason polymer lowers are best suited to drop-in trigger assemblies. All the spring pressure is contained within the metal frame housing the trigger assembly.
With a drop-in trigger, this is dead even. With a you-assemble-it kit, the advantage goes to metal.
An AR upper mounts to a lower at the front and rear takedown points. These are also weak points on both kinds of lowers. The polymer lower can warp or break. A metal takedown point can break. Twisting the upper when it is connected to the front pin is a common way to damage a lower. Polymer is much easier to damage. Sometimes this damage can be repaired, but it will always be weaker at the repair than the original.
ARs are designed to swap uppers. You can go from a .223 with a 16-inch barrel to a .350 Legend with a 20-inch barrel in seconds.
If you want to swap uppers regularly, go with metal.
AR lowers are made of metal or polymer. You can coat the metal or paint it. Both scratch. Polymer lowers come in every color you can think of and, depending on the maker, can be multi-colored. Scratch it and the color is still there.
The clear advantage goes to polymer.
Guns have two enemies, rust and politicians. Aluminum will rust, but not as fast as steel or iron. Polymer does not rust. Regardless, if you take care of your gun, rust will not be an issue.
This may sound like the advantage goes to polymer, but an AR is more than just a lower. It is the internals, barrel, upper, detents, pins, buffer and spring. These steel and brass parts are subject to corrosion and will rust sooner than an aluminum lower.
For rust, this is a tie.
The jury is out on how long polymer frames will hold up to ultraviolet (UV) radiation. We do know polymers will break down eventually when left in the sun.
Since the makeup of the polymer varies from manufacturer to manufacturer, the length of time it takes for the material to break down in the sun varies. Metal will not break down. Coatings can fade. The metal remains.
The advantage goes to metal.
BUT POLYMER HANDGUNS
At this point, someone is going to bring up polymer handguns. Here is the reality. Polymer handguns are a new creation. 1911s made 100 years ago have worn out a bunch of barrels and still function. Will Glocks, for all their durability, last that long?
Another consideration is pressure. Long guns build up a lot more pressure than handguns. In both kinds of guns, this pressure is the reason the barrel is made from metal. Metal frames can handle higher pressure than polymers.
Durability is the third factor. Metal is just tougher than plastic. While you might wear out a barrel or two with a polymer lower, a good metal lower will last for many barrels and many trigger sets.
Which one is the best? A new shooter won't know the difference between the two. Serious shooters who will run cases of ammo in a year need a metal lower. Casual shooters can get polymer but in most cases we prefer metal. If you have specific reasoning for choosing a polymer lower it can be an option, but overall a good quality metal lower reciever from a reputable company is going to be your best bet. If you need help making a decision, the experts at X-Ring Supply can guide you to the right choice.
A polymer lower. This was milled to the exact specifications provided by the company, using the jig set they provided. When installing the hammer and spring, the retaining pin ripped through the wall of the lower. A small flap of ripped polymer can be see over the hole, proving this is a not a drilling accident. Whether or not this lower can be safely repaired, remains to be seen. Chances for a safe repair are low and it will certainly never handle heavy recoil cartridges.