Quality Risks With Polymer Lowers

Nowadays, polymers are extensively used in the firearms industry. There is a myriad of reasons for this, which we’ll examine here. But what are the downsides and the upsides when we’re talking about polymer lowers?

The Early Days

Most people think that Glock pioneered polymer-framed handguns. However, Glock wasn’t the first—it was Heckler & Koch with their VP70 9mm pistol. That was back in 1970, years before Glock tried it.

H&K VP70
HK’s VP70 was the very first polymer firearm, beating the Glock by years. (Photo: Wikipedia)

However, Glock was the pistol that took off like a rocket ship. Of course, at first, the new “plastic” handgun wasn’t well received, as people were certain that it wouldn’t hold up, and that it would fail miserably.

Even before Glock, the M16 featured a number of plastic parts. The rifle certainly had its share of detractors who didn’t trust it. And it’s no wonder—it was coming in on the tail end of generations who had been issued rifles made from steel and wood. The M1 Garand and M-14, both heavy rifles made from traditional materials, were standard issue for everyone leading up to and into the Vietnam era.

Guys who were used to heavy rifles were being issued a lightweight rifle in a smaller caliber, constructed partially of lightweight plastic. It’s no wonder they nicknamed it the “Mattel Toy.” As an aside, I actually handled one of the very early AR-15s that had been issued in Vietnam. It literally did feel like a toy!

Benefits of Polymer

First, let’s take a look at some advantages.

  • Less Expensive
  • Lightweight
  • Durable
  • Customizing
  • Pliability

These days, the cost is a major factor for many people who purchase firearms. It’s a fact that lower-cost firearms sell better than very expensive ones. AR-15 lower receivers made of polymer cost about one-quarter to one-half of what a machined aluminum receiver would.

Another fact is that people like their firearms to weigh as little as possible. Lighter firearms can be carried further. Plus, they’re more pleasant to hold.

When it comes to durability, polymers have a certain amount of flex to them, as they don’t tend to dent or be crushed the way aluminum might. In that aspect, durability is a plus with polymer, and we all love durability. Having firearms made from tough substances is a major advantage.

Polymer AR receiver.
Aside from being lightweight, any color imaginable can be added to polymer, which adds to the customization factor. (Photo: GunMag Warehouse)

Nowadays, people love to customize their firearms and make them unique. And who isn’t sick of black rifles? With polymer, the receiver can easily be made in any color that’s desirable. Color changes are easy. Pink, Flat Dark Earth, and Olive Drab, they’re all easy since the color can be mixed in during the manufacturing process. And unlike anodized or painted finishes that can scratch off or wear away, the polymer colors won’t do that. Not only can the wearing away of the protective finish look unsightly, but it can also lead to corrosion for certain metals.

As an aside, people sometimes mention that the finish being scratched off of aluminum receivers can lead to corrosion. But let us all recall one detail about aluminum: it cannot rust.

Weak Points of Polymer Lowers

Despite all of the benefits of being constructed with polymer, a polymer lower does have a few weak points. Namely, the buffer tube area, the hinge pin area, and the magazine well.

Buffer Tube Area

Polymer lower receiver.
The area where the buffer tube attaches can be easily damaged during installation. (Photo: Shooting Illustrated)

With an AR-15 that has a polymer lower, a weak point is where the buffer tube screws into the lower receiver. Although the 5.56mm is not a high-recoil round, stress is still exerted where that buffer tube attaches to the receiver. Not just from the rifle firing, but also from handling it. The stock sticks out, which creates leverage when the rifle is handled roughly.

Some companies have beefed up the area where the stock attaches to the receiver because when tightening the nut in that area, the receiver can be snapped or marred, rendering the entire piece useless. Builders are cautioned not to overtighten the nut during installation because it can ruin the lower receiver.

Hinge Pin Area

The hinge pin area where the upper and lower receivers connect, along with the rear takedown pin, also can be a problem area with flexing. The pins can either be too tight or too loose on some of the polymer lowers. Also, it’s been noted that steel pins sometimes walk out of their holes during firing. Apparently, the polymer’s flexible nature doesn’t mate well with steel pins.

There are reportedly at least some polymer AR lowers that have failed. At this point, we wonder if this is a widespread phenomenon or a limited number of cases that are receiving a lot of attention.

Magazine Well

Because the specs vary from lower to lower, magazines sometimes do not fit as they should. Some mag wells are too tight, others are too loose. The drawback here is that since polymers are not quite standard, it can interfere with mags, pins, and parts.

Catastrophic Failures

We sometimes hear stories about the polymer lowers “exploding” or otherwise catastrophically failing. Granted, these events are likely not very frequent, and accounts of them could very well be overly publicized. The stress that lowers experience during firing is really pretty low, especially with 5.56mm rounds.

Exploded polymer lower.
An exploded AR-15, which doesn’t happen with frequency. Note that the upper receiver has damage as well, so this could have possibly not been the fault of the polymer lower, but some other cause. (Photo: The Firearms Blog)

Other Considerations

Some lowers are so flexible that if pressure is exerted, the upper and lower can actually experience a gap. This comes from the lower flexing drastically. Shooters notice this most when tightening up a sling to use as a shooting aid.

A drastically flexing polymer lower would cause a marked shift in bullet impact since the barrel would literally be pointing in a different direction than the one it was zeroed in. Rigidity is the friend of accuracy, and polymer is not rigid. I’ve heard accounts of polymer lowers flexing dramatically just from being grabbed and pressure exerted upon them.

Aside from accuracy problems, reliability issues could arise. If cartridges cannot line up with the chamber as they feed from the lower/magazine well, then they will not properly feed.


It’s been pointed out that the SCAR platform has a polymer lower, which is absolutely true. And it works very well, from all reports. However, let us note that the SCAR was designed from the ground up to have that polymer lower. A big reason for its success is likely that there was no change over from aluminum to polymer, and hence, no learning curve.


AR-15 lower receivers typically and historically use aluminum, which has several advantages. It is lightweight, affordable, and available. It seems that aluminum will offer more rigidity and durability than polymer will.

Not to mention, manufacturers have the process of making aluminum lowers down pat very well, as that’s the way it’s been done since the design came out decades ago. “If it’s not broken, don’t fix it.” And there’s something to be said of the tried-and-true methods since they do work.

While polymer certainly is lightweight, it isn’t that much less weight than the good, old aluminum lowers.

A Few Camps

It seems as if there are a few camps of shooters when it comes to polymer and aluminum.

The first group accepts polymer and is moving ahead with using the lower receivers for builds or are purchasing ARs that are already built with them. They tout the weight and cost savings as advantages.

Another camp is undecided. They may be cautiously optimistic, but not quite sure yet where this concept is going and if it will be widely accepted.

Finally, there are the purists who will simply stick with the aluminum lowers because they’re proven and really don’t weigh much more than polymer, to begin with.

Are we moving backward?

It just occurred to me that people are pining for lightweight AR-15s. Ummm….hello?

The Vietnam-era AR-15 that I handled really did feel like a toy, and I say that with no exaggeration. And for good reason—Eugene Stoner designed it that way. It was intended to give the soldier a lightweight, short assault rifle that could be more easily carried than a battle rifle, along with more ammunition.

Then came the CAR-15 way back when, which was even more compact and lighter.

Somehow, over the years, we’ve gravitated away from that lightweight concept. There are a few reasons for these steps backward.

The first is “technology.” Advances in the latest gadgetry are to blame here. I’m talking about lights, lasers, optics, rail systems, and a partridge in a pear tree.

Yes, lasers are neat. And in some very limited venues, they do have a place. Especially in Special Operations where they use IR lasers that are invisible except to those using NODs (Night Observation Devices). Enemy combatants can be “painted” with the lasers and they don’t even realize it. I get that.

Scopes and RDS (Red Dot Sights) of all varieties have blossomed in the market. There are certainly some amazing optical setups nowadays! They can extend the range of the AR-15 dramatically, as well as make close-range engagement faster. And dot sights these days are getting tinier and tinier as technology advances. But these items still do add weight.

Many folks are using a red dot sight along with a magnifier. This combination gives us the best of both worlds to choose from at a moment’s notice. But again, we’re adding weight.

And lights—we can’t forget lights! Sure, we have to be able to identify what we’re firing at during low-light conditions. But again, lights add weight, just like everything else.

Vertical foregrips? More weight.

Oh, we can’t forget suppressors! You’re just not a cool guy unless you’re running a suppressor. Hell, we see them on lever action carbines these days, so our AR certainly has to have one.

Last, but not least, is the myriad of rail systems that we have to have in order to mount these gadgets to our “lightweight” carbine. And forgive me if I’ve left out any accouterments/add-ons for the AR-15, I was just going off the top of my head on the accessories list.

All of these items add weight, and it’s no damn wonder that we now have weight issues with the AR-15/M16/M4 system of weapons.

And there’s another thing that strikes my funny bone. The same people who tell me the M1 Garand weighs far too much to make it a serious fighting rifle for modern times, have no problem dressing up their AR platform by bolting on so many gadgets that it weighs just as much as the Garand.

Forgive my rant, but our infatuation with technology can drive me nuts at times. I half expect to see an M4 mounted on a chassis with two wheels and a gun shield, similar to what the old Russian Maxim machine guns of WWII used to look like.

Teething Pains?

Polymer lowers seem to be somewhat in their infancy, having just come out in the past few years. Naturally, there will be some bugs to work out. However, are these just detail-oriented bugs that can be ironed out over time, or is polymer truly not a great material to use for the AR platform?

At this point, it seems that only time will tell.

An AR polymer lower.
Polymer lowers are far from being standardized and a combination of plastic and metal innards can be found in various lowers. (Photo: Shooting Illustrated)

I’m definitely not an anti-polymer guy, as I own handguns with polymer frames. However, those have been around for a very long time now and the kinks have been ironed out.

Final Word

Are the weight and cost savings worth it? Is the frustration of fitting parts worth it? Only the person building the AR or purchasing the built rifle can decide that.

In the future, I’ll wager that polymer will become more prevalent in AR lowers as the kinks are worked out. Just like polymer-framed handguns, there is a learning curve to making them.

As for me, I’ll stick with good old aluminum for now, as it’s tried and true. As I said, if it’s not broken, don’t fix it.

How about our readers? Perhaps those with experience with polymer lowers would like to chime in.

Jim Davis served in the PA Dept. of Corrections for 16 ½ years as a corrections officer in the State Correctional Institute at Graterford and later at SCI Phoenix. He served on the Corrections Emergency Response Team (CERT), several of those years as a sniper, and also the Fire Emergency Response Team (FERT). For 25 years, he was a professional instructor, teaching topics including Defensive Tactics, Riot Control and Tactical Operations, Immediate Responder, and cognitive programs as an adjunct instructor at the DOC Training Academy. He was then promoted to the title of corrections counselor, where he ran a caseload and facilitated cognitive therapy classes to inmates. His total service time was close to 29 years. He was involved in many violent encounters on duty, including incidents of fatalities. He is a dedicated Christian and attributes any skills that he has to the glory of God.

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