Polymer. It’s all the rage and has been since 1982. We will widely accept polymers in our handguns and even some of our rifles. A lot of modern rifles use polymer lowers. Guns like the FN SCAR and the Bren are embracing polymer. ARs, on the other hand, have been largely polymer-free. There have been attempts to produce polymer ARs, but they’ve faced several challenges. However, KE Arms might have mastered the polymer lower, and they’ve done it in both 9mm and 5.56. I’ve recently got my hands on the KP-9, 9mm lower, and have been giving it a spin.

KE Arms didn’t invent the polymer lower. In fact, the origin of their lower traces back to a company called Calvary Arms, which is no longer in business. Cavalry Arms produced polymer lowers from 2000 until it closed in 2010. In 2003, it was more or less mastered with the CAV-15 MK III. When Cavalry Arms closed, they sold their proprietary information to GWACS.

The weapon, lower, and information ended at KE Arms, with a vengeful GWACS later suing the company. That’s a story unto itself. It’s full of drama, an anti-gun infiltrator, and more. It’s a long story, but it seems like KE Arms pulled ahead, and in 2023, the Judge vacated the claim filed by GWACs.

The KP-9 – How It Works

Historically, the problem with polymer AR-15 lowers comes from the portion where the receiver extension attaches to the lower. This area often breaks. Companies like ATI have used metal and created hybrid lowers to try to prevent this. KE Arms went an entirely different route. They used an injection-molded design that doesn’t stop at the lower.

They mold the stock, receiver extension, and pistol grip, which are made of 30% glass-filled nylon polymer. The molds create two halves of the lower, which are sonically welded together. I didn’t know anything about glass-filled nylon, so I looked it up. Adding glass to nylon obviously makes it a bit tougher.

Combining the two also creates a material with better dimensional stability, increased wear resistance, and better temperature resistance. Making the stock, buffer, tube, and beyond out of nothing but polymer eliminates potential weaknesses.

The benefits of a polymer lower receiver are a lower weight and potentially a lower price point. The receivers on the KE Arms website retail at various prices, depending on the configurations. I purchased mine for $59.99. That’s not a bad price for a partially stripped lower, a stock, and a pistol grip.

Partially complete means it comes with the ejector and magazine release installed. It also comes with proprietary takedown pins, selector detent, and the roll pin that retains the buffer detent. Other than that, I just ordered the KP-9’s parts kit to finish it out.

Going Over the Lower

The lower uses a simple straight blowback system and requires a carbine spring and a heavy buffer. The KP-9 comes with an ejector, which can be adjusted if necessary. This can be handy for tuning the gun for reliable and proper ejection with your upper and BCG. The KP-9 utilizes Glock magazines and has a flared magazine well for those quick reloads. It’s almost as good as a Colt SMG magazine lower.

The KP-9 uses the M16A1 length of pull. At 13 inches, it’s perfect for the vast majority of shooters. You might hem and haw about an adjustable LOP, but 13 inches will work with most shooters without a problem. In the end, we get an M16-style butt plate with a little extra texture. The stock also features a few sling points and options.

The stock has QD cups, so installing a QD sling design is possible. There is also a slot that allows you to run a sling through the stock, or you can attach an M-LOK sling point here since the slot is an M-LOK spec design.

In terms of weight savings, I weighed the KP-9 against a dedicated Aero Precision EPC lower, both with identical buffers. The KP-9 weighs one pound and fifteen ounces, while the standard lower weighs two pounds and eight ounces. That’s a decent little bit of weight savings. You can feel the weight savings, and the gun feels front-heavy. I used a Mutt AR-9 upper with a rifle-length barrel, STNGR handguard, and standard upper.

The takedown pins are nonstandard and do not attach to the lower like a typical lower. These are fully removable and remain in the lower via detents. It’s odd, but it works, and my advice is not to lose your pins.

To The Range

Ergonomically, the lower has quite a few high points. It’s lightweight, and the stock length is just perfect. I find the comb comfy and perfect for using optics. One thing I’m not a huge fan of is the pistol grip. The grip is fine, but where it meets the stock annoys me. It’s annoying, but it doesn’t detract from my shooting ability.

It took me a while to get used to the safety. It doesn’t click into place. If you’ve shot standard ARs, this might short-circuit your brain just a bit. Again, it’s not a problem but an ergonomic oddity. The KP-9 excels when it comes to the safety. It’s a clever, low-profile, ambidextrous design that works insanely well. I ran the KP-9 PCC both left and right-handed.

While I looked like a shaved ape trying to manage the gun left-handed, I could use it left-handed. The flared lower also helps you manage quick reloads and seems to take the abuse well, especially if you are trying to run the gun left-handed and slamming the magazine into the magwell like an idiot.

The included Mil-Spec trigger functions well. It’s light and short, and the rest is extremely chunky. I don’t ride the reset, but I can hear and feel it as I shoot. Another noticeable noise is the spring reciprocating with the buffer. It’s loud but does not affect the weapon’s reliability; it’s just somewhat annoying.

Recoil seems to be a little less violent than that of most other straight blowback 9mm AR rifles. Maybe the plastic slightly flexes, much like a polymer frame handgun? It’s tough to say, but there is a slight, noticeable difference.

Going Hard

I’ve been using and shooting the KP-9 for several months now. It’s been perfect and reliable with all types of ammo. I had concerns, especially about the takedown pins. They’ve yet to fail me. The pins holding the trigger in haven’t failed or seemingly wallowed out the receiver. It might be polymer, but it’s tough, and this might be the only real way to produce an entirely polymer frame lower receiver.

KE Arms has it worked out quite well. While I can’t adjust the stock or switch the pistol grip, I can get an almost complete AR 9 lower for $60, and that’s tough to beat. If you’re on the fence about polymer lowers, check out KE Arms; they do it right.