I’ve only had negative experiences with polymer AR lowers. It seems like it would be easy to conquer, but here I am, deep into the world of polymer lowers, and I only use them for rimfire purposes. When I saw the KE Arms KP-9 lower at a local pawn shop for forty bucks, I hesitated, but it was only forty bucks and came complete with a stock. I enjoy pistol-caliber carbines, and I took the chance.
Like most of us, I know about KE Arms through Sinistral Rifleman, Forgotten Weapons, and InRange TV. Specifically, the What Would Stoner Do project. Or, if you follow court cases, KE Arms has to defend itself against Calvary Arms and an anti-gun investor trying to shut them down for producing the KE-15 lowers. I settled on the KP9 lower and eventually built a 9mm platform.
The KP9 In Depth
This is a lower receiver review, so I won’t talk much about accuracy or reliability unless it concerns the lower. This all-polymer lower attempts to fix the problems with most AR lowers by making the lower receiver extension and stock one solid piece. This eliminates the weak point where the receiver extension attaches to the lower, often a weak point that turns polymer lowers sour.
If you’re addicted to adjustable stocks, the KP-9 isn’t for you. It uses a fixed stock that’s the same length as an M16A1. To me, the length is just right. The stock has a QD cup for skins and an M-LOK-sized slot for mounting a sling point or running a sling through the slot from one side to the other.
The KP-9 uses an injection-molded design made from 30% glass-filled nylon, giving it some unique features. The stripped lower comes with the proprietary parts needed to put things together. For example, it doesn’t use your normal takedown pins; you have non-captured pins.
In total, your stripped KP-9 comes with the following:
- Pivot and Take-Down Pins
- KP-9 Selector Detent
- Buttplate
- 5/64″ Roll Pin to Retain Buffer Detent
- Ambi Glock Magazine Catch Assembly
- Adjustable 9mm Steel Ejector
The KP-9 works mostly with standard lower parts kits and can use MIL-SPEC trigger systems. However, you do have to use a carbine buffer spring and a heavy 9mm buffer. You can use a standard hammer pin, but KE Arms produces a longer one. I purchased a KE Arms lower parts kit to simplify my life, which all snapped right in.
The KP-9 – To The Range
The KP-9’s ergonomics are fairly solid. The stock length is fine for me, and the stock itself offers a solid and comfortable cheek weld that’s super beard-friendly. The point where the stock meets the pistol grip just feels odd. I want my hand to go higher but can’t get it higher. It’s odd, but it doesn’t necessarily affect my performance.
The controls are easy to reach, and the magazine release is surprisingly ambidextrous. That’s quite nice with Glock magazines, which have already changed your arms manual. The ambi magazine release allows your thumb to depress it as you grip the magazine. This makes it easy to reload with retention.
The rest of the controls are as easy to reach as any other AR option. The safety is fine, but it’s soft. It doesn’t click or snap when pressed from safe to fire and back again. This isn’t terrible, but when you are used to that snap of safety, it’s a bit distracting.
The magazine well took full advantage of the polymer design and has a massive magazine well that makes pushing the magazine inside the gun easy. Reloads with the KP-9 are rapid and intuitive. The magazine also drops straight out with the press of a button.
The spring provides a very loud and audible boing with each shot. It reminds me of the M16A2 I had in boot camp. The lower is set up for direct blowback. However, I don’t see why it couldn’t work with a radial delayed design, but don’t quote me.
Running and Gunning
What are most PCCs used for these days? Especially full-sized PCCs? They are used for plinking and competition in most cases. The KP-9 seems to lend itself well to the world of PCC competition with the big magwell and ambi magazine release. The stock is comfy, making getting behind the gun and the optic easy.
While it does deal with blowback recoil, it seems to do so well. The KP-9 doesn’t beat you up, and the heavy buffer keeps things well balanced and the recoil timid. It’s not as bad as other 9mm blowback guns, and maybe the fixed rigid stock helps.
You can gun through magazine after magazine, and the KP-9 with my home brew upper remained reliable.
Polymer Finally Works
Polymer AR lowers can work, but it does seem best to build them like the KP-9. It eliminates the weak points, shaves weight, and feels quite nice. There are some obvious downsides and a loss of customization, but the design holds up and can be extremely affordable. It might not be for everyone, but as a guy who likes things that are a little different, I like the KP-9.
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