Polymer AR-15 lower receivers offer light weight, surprisingly high strength and, most importantly, very low cost. Historically they’ve been a bit of a mixed bag in the quality department, mostly due to fit issues caused by molding imperfections. StarLos Firearms has set out to improve on past offerings from other manufacturers by improving the polymer and improving the manufacturing process, and we were happy to put their SLF-15 complete lower through the wringer to see if it worked. 500 rapid-fire rounds? Check. Run it over with my truck? Check . . .
My go-to FFL, Best Buy Surplus, intends to be one of the largest stocking distributors of these lowers in the country. At an MSRP of $119.99 for a complete, ready-to-run lower and $59.99 for a stripped lower, they’ll be some of the least expensive options anywhere. The guys at BBS asked if I would provide an honest review ahead of them stocking these things in quantity, and even donated 500 rounds of Federal American Eagle .223 to the cause. If you like what follows and need an SLF-15 in your life, call BBS (509.535.5375) for the best pricing. They’re happy to ship to your FFL.
The receiver itself looks and feels really great. It seems stronger and is definitely much cleaner and more precisely manufactured than the New Frontier Armory lower that I’ve used as a desk ornament and pen holder for a couple of years. This met expectations, as improving the fiber reinforced plastic formula was a primary goal of StarLos. To improve fit and finish, the lower isn’t actually molded into its final form. Rather, billets of polymer are made in the rough receiver outline and then the lowers are CNC machined from those.
So while the stripped lower appears about as good as it gets in plastic fantastic world, I had serious reservations about a fire control group, magazine catch, takedown and pivot pins, and receiver extension (buffer tube) made entirely of polymer.
StarLos is definitely not the first to do a 100% polymer AR-15 hammer, trigger, and disconnector, but the whole idea of this has seemed so absurd to me that I’ve avoided them like the plague. At this point I suggested to Best Buy Surplus that a review of the lower would be meaningless without putting a bunch of rounds through it, and thought maybe StarLos would donate 500 or even 1,000 gun food units. No worries, said Bill of BBS, setting a box of AmEag on the counter. Hmm, nice.
Well, I begrudgingly had to admit that the trigger pull actually felt good. Much better than most parts kit or otherwise “mil-spec” AR triggers. Less creep and significantly smoother than parts kit, with a pretty clean break at just over 4 lbs. The lightweight hammer in combination with a full-power hammer spring is supposed to ensure reliable ignition of even stubborn primers and a quick lock time. Yeah, but do the sear engagement surfaces hold up? More on this later…
Dimensionally, almost everything appears to be in spec. My go-to upper from Adams Arms bolted right up, every magazine type I have functions properly, and aftermarket trigger units, safeties, and grips all went in cleanly. The only exception I found is a nub on what’s normally the flat side of the safety bar. It’s designed to work with the polymer trigger, but would have to be filed off to work with a standard replacement trigger.
On The Range
One 40-round and two 30-round magazines accompanied me to the woods along with the 500 rounds of AmEag and the SLF-15 wearing an Adams Arms hat. I figured I’d mag dump 100 rounds from those three magazines, take a few minutes reloading while the rifled cooled down, and repeat until all of the ammo was gone or some sort of parts breakage occurred.
The result was 500 empty cases. That plus 150 rounds of 9mm and I had a good, if noisy, Sunday morning.
However, it wasn’t all smooth sailing. In the video at top you’ll see my usual close-up of the trigger pull and reset, done before I had shot the gun. After one of the pulls, while releasing the trigger slowly to get a good view of the reset, it never came. The disconnector didn’t pop off the hammer. This same issue reared its head maybe 10 times while shooting. To fix it on the range, I pushed forward on the back of the trigger or just pulled the trigger back again and flicked my finger off it.
Basically, it wanted to stick some of the time if I let it out too slowly. Keep in mind I removed the FCG to take “before” photos of the sear engagement surfaces and, while all of the parts other than the disco (to include trigger pins, safety bar, etc) were as dry as the Atacama and the trigger fit was a touch snug in the receiver, I chose to shoot it exactly as I got it. As I later confirmed, some lube and a wee dab of judicious filing would have entirely prevented this problem.
Speaking of sear surfaces, I truly figured they would wear away and round off pretty much immediately, which is why I did “before” and “after” photos. I’m shocked to report that, after 500 rounds — including a couple mags of bump firing that almost cost me my camera haha — I cannot see any difference in the hammer/trigger or hammer/disconnector engagement surfaces whatsoever. Not even the slightest bit of change. But seeing is believing:
So, color me surprised. And impressed.
Unfortunately there was one more problem near the end of the shooting session. The castle nut worked free and, while trying to determine why the bolt suddenly started sticking in the receiver extension, I spun the entire stock. This caused the buffer retainer and spring to pop out and the receiver plate to spin, which caused the takedown pin detent and spring to head for the hills.
As mentioned, the loose stock initially caused a hangup on the bolt carrier, but once the buffer retainer dropped into the FCG pocket it actually ran fine until all 500 rounds were downrange.
Aside from these two issues, the receiver worked smoothly. The safety functioned crisply and properly, the bolt catch did its thing before and after every magazine change, the mag release worked smoothly, the grip stayed attached, and the receiver pins didn’t even explode.
Really, everything still looks new.
Oh, yeah. I also ran it over with a Nissan Xterra. The whole thing’s in the video, but…
It got dirty but otherwise didn’t care one bit.
My experience with the SLF-15 complete lower wasn’t perfect, but I still think it has a place. The issues it had weren’t due to use or due to parts failures — they weren’t due to the parts being plastic — but due to poor assembly. In fact, the magazine catch needed another rotation before it was properly installed. No, not exactly confidence-inspiring but had I checked the castle nut and lubed the FCG like I wanted to, I’m fully confident it would have sailed through the 500 rounds without a hiccup and kept on going well into ammo-induced indebtedness.
I can also see picking up a handful of stripped lowers for SHTF or some sort of gun grab hedge investment. They’re so affordable, they can be a backup to the backup. Heck, the polymer’s supposed to be impervious to everything so bury it in the ol’ backyard and it should be right as rain even if the zombies don’t come till 2073.
Specifications: StarLos Firearms SLF-15 Complete Lower (copy-paste from SLF website)
- LIFETIME WARRANTY
- Winter / Oversize Trigger Guard
- Works on any Mil.Spec Upper Receiver
- Multi Caliber Supported
- NEW billet Design
- Extremely high chemical resistance
- 4.7oz for a custome light weight AR 15 build
- High tolerance to heat
- Color is black
- Carbine Buffer and Spring
- Steel Castle Nut
- Steel Locking Ring
- 6 position collapsible commercial M4 style stock
- SLF 4lb trigger group polymer
- 2.6oz commercial polymer buffer tube
- Standard A2 Black Pistol Grip
- DOES NOT COME WITH PMAG
Made In USA. The SLF-15 Weighing in at 25oz with the polymer trigger group and 2.6oz polymer buffer tube make this the lightest complete lower on the market today,The SLF trigger group has been heavily tested and despite heavy carbon build up after thousands of rounds the 4lb trigger maintains its crisp, fluid pull all the way through. Corrosion resistance of FRP is a function of both our resin content and the specific resin used in the laminate. The SLF trigger group is also back with a LIFETIME WARRANTY.
Ratings (out of five stars):
Ergonomics: * * * *
Three stars is average, and this is a standard AR lower with standard grip, standard stock, standard controls, etc. So, average there. Plus one star for extreme light weight and noticeably better than parts-kit trigger.
Reliability: * *
I’m actually totally confident that this setup is reliable for thousands upon thousands of rounds if assembled properly. If the trigger group was lubed and fit a little better and the castle nut was properly tightened, this rating would have been totally different. On the plus side, there was none of the extreme wear I expected to see on the polymer trigger parts. In fact, there was really no wear visible at all. The polymer hammer does solidly dent primers, and the buffer didn’t even shoot out the back of the polymer buffer tube and impale my shoulder. The polymer that the receiver is made out of is extraordinarily strong and resilient, and may even hold up to more abuse than aluminum.
Customization: * * * * *
Well, it’s a dimensionally in-spec AR-15 lower receiver so you can do whatever the heck you want to it. Or, of course, stripped lowers are available for half the price and can be built out from scratch.
Overall: * * * *
Rated against other polymer lowers that I’ve owned and shot, the StarLos lower is noticeably nicer. The quality of the polymer is clearly better — it’s stronger, denser, harder (yet still flexes rather than cracks) — and the machining and tolerances are better. Despite this, the price remains at rock bottom. Minus one star because of sloppy assembly. If I were buying an SLF-15 complete lower, I’d give it a thorough once-over.
[Edit] Guess what? It passes “the push-up test.” If you must know, I’m 6-foot, 190…
The buffer tube flexes a little but the receiver itself holds pretty firm. I think with the stock all the way out, the buffer tube would kink or crack. But I’m not testing that yet, as I’m hoping to measure how much force of this sort is necessary to actually crack the receiver in order to compare that to the amount of force required to do the same thing to a standard, forged aluminum lower.