TTAG’s readership is a well-informed crowd. So I’ll spare you an explanation of the purpose(s) of an eighty percent lower. I will say, however, that if you’re not making at least a portion of your (where legally permitted) DIY firearms each year you should strongly consider doing so.
At the very least, it’s a great way to further your continued learning about the mechanics and maintenance of firearms. The better you know your tools — both those on the workbench and “on your hip” — the easier it is to deploy, employ and repair them, if needed.
When it comes to eighty percent lowers, polymer is a popular choice. The finishing process is less-intense than aluminum (think hot knife cutting through ice cream versus cutting though an ice cube). Polymer lowers tend to be lighter and less expensive than aluminum. However, there have been more than a few instances where polymer frames have failed due to poor material choice or design.
The folks at James Madison Tactical (JMT) have decades of experience in aerospace engineering and manufacturing. They have a range of skills and knowledge easily transferable to the firearms industry, as we’ve witnessed through their Saber Drop-in Trigger.
JMT ‘s AR-15 80% Polymer Lower (Gen2) is a prime example of intelligent humans thinking critically to produce a solid product. Gen2 utilizes the same special composite/polymer blend as Gen1, lending itself to increased durability while remaining easy to mill.
On its product webpage, JMT calls out nine features of the Gen2 lower receiver:
1) Enhanced buffer tube mount
2) Grated magazine well matched for PMAGS
3) Textured areas on lower receiver
4) JMT markings
5) Flanged magazine well
6) Grated front finger hold
7) Oversize trigger area
8) Boxed longerons on left and right side of fire control pocket
9) Threaded brass [grip screw] insert
While the many of the Gen2 upgrades are cosmetic refinements, there are also several design changes that increase compatibility and strength: extended upper fire control box sections, upper left magazine well box section and extended bolt release roll pin posts.
It’s worth noting that JMT’s manufacturing process calls for the buffer tube mount to be molded around buffer tube threads. So instead of threading the buffer tube mount post-molding, they simply unscrew the buffer tube thread portion of the mold once it cools. Aligning the buffer tube thread mold to the overall mold helps to ensure more precise placement of the threads.
The JMT Gen2 lower comes with a polymer machining jig. The jig’s designed for one-time use and fits the Gen2 lower perfectly. The polymer jig surely isn’t as forgiving as a steel jig, but it comes at no additional cost; if you prefer to use an alternative jig it’s no loss.
When using the included JMT polymer jig, I recommend using vice plates and paying close attention to any flex in the jig due to over-tightening, which could potentially lead to axis misalignment.
Using a drill press with manual X/Y axis controls, I got to work on the remaining “twenty-percent,” following a fairly standard process that begins with the trigger pin, hammer pin and selector holes.
As it was my first time using a polymer jig, I admit I was nervous. But each of the three presses of the appropriate bit perfectly hit their mark on the other side of the jig.
After flipping the jig ninety-degrees, ensuring the press was set to the appropriate RPMs, and triple-checking axis alignment, I began the laborious (and rewarding) process of hand-milling the fire control pocket, trigger pocket and trigger slot.
I quickly determined the limits of the material/bit/RPM combination. I worked the bit methodically within the confines of the jig. Pass by pass, I removed the unwanted material from the two main cut areas, then finally pushed through to create the trigger slot.
Working with JMT’s composite/polymer material was easy. No splintering, cracking or melting. The material cuts cleanly.
The detailed yet simple machining instructions provide the information needed to keep the user on-track. Due to its contrasting color, the jig is also a good platform for feedback.
If I saw light grey remnant at any time during the finishing process I knew I was cutting outside of specifications, or drilled completely through the lower. Compared to a steel jig, you don’t risk damaging your bit or kicking the jig out of place. A downside: you can easily cut through the jig, permanently removing critical material from the lower.
Once the milling was finalized to specifications, I took the new “firearm” over to the workbench to fit the hardware and mate the upper. Working my way through the orderly installation of components I encountered a variety of fit tolerances.
The molded buffer tube threads, pre-milled bolt catch and pivot troughs, and pre-drilled buffer detent, pivot/takedown pin detent, bolt catch plunger and mag catch spring holes all fit well with their appropriate components. The brass grip screw insert is a great touch; it provides added security for a wide range of grips that fit to the lower.
I encountered zero issues across a selection of drop-in triggers, anti-rotation pin sets, selectors, end plates (standard and QD) and buffer tubes (with and without ears).
There were a few surprises. The takedown and pivot pin holes are slightly oversized, allowing the pins to wobble more than I’d like when they’re in the “out” position (no issues when assembled).
The magazine release trough and magazine release button hole were under and over-sized, respectively. I removed material from the magazine release trough to allow a Mil-spec magazine catch to fit properly, then removed a little more to make an ambidextrous magazine catch fit perfectly.
Several ambidextrous magazine catches are not compatible with the JMT Gen2 lower, such as the Troy ambidextrous magazine release. All magazine release buttons I tried resulted in a sloppy fit. Angled buttons may require the removal of a small amount of material from the frame of the lower in order to function reliably.
Mating the lower to its 7.5″ Spinta Precision/Trinity Force/Cross Machine Tool .223 Wylde upper was rewarding. Finding no play at the pivot pin, I removed a minute amount of material from the sides of the CMT upper’s takedown pin hole post (mostly coating). The two complete halves locked-up well. Zero wiggle, nor waggle.
On the range, I dumped over five hundred rounds of CapArms .223 Rem 55-grain FMJ and 69-grain SMK through the platform, over an afternoon spent testing various buffer systems and tubes, gas systems, barrels, and BCGs.
The lower integrated well with each change. All mags fed consistently, bolts locked-back and tolerances remained. Altogether, the JMT Gen2 lower shows no signs of weakness.
Despite its angular profile, the Gen2’s oversized trigger guard is quite comfortable. The ribbing on the front of the magazine well and flared squared-edge magazine well lip provide lots of potential for positive contact and control.
James Madison Tactical’s Gen2 AR-15 Eighty-percent Composite/Polymer Lower is an excellent balance of technical, functional and aesthetic considerations.
The lower is simple to finish. It proved to be a reliable mix of material and solid engineering. Its design lends to compatibility with most AR-15 parts.
While there are a few areas are that could use another look and possibly some adjustment, the JMT Gen2 AR-15 Eighty-percent Lower is a great option for your next build.
Specifications: JMT AR-15 80% Polymer Lower (Gen2)
Price as reviewed (Gen2): $99.95 MSRP
Colors: Black, Wolf Grey, ODG, FDE
Ratings (out of five stars):
Design: * * * *
JMT’s Gen2 lower is simple to finish, feels great to shoot, and is visually pleasing, to boot. The magazine well is sculpted to match Magpul magazines, flows well with the other lines of the design, and will appeal strongly to PMAG fans. There are several areas that could be improved, but they are not function-critical.
Jig / Finishing Process: * * * * *
The included finishing jig is more than adequate for completing the “twenty-percent tasks.” While the jig can be flexed, I never found the jig to actually flex in the vice or create any issues. The finishing process itself is clearly outlined step-by-step with very nice diagrams. The composite / polymer material is forgiving and produces a smooth finish.
Durability: * * * * *
The composite / polymer material utilized in the Gen2 lower, combined with precision engineering, makes for an extremely durable product. The lower can take a beating from high-pressure uppers, holding-up better than most polymer alternatives. The material also seems to resist scratches quite well.
Compatibility: * * * *
While most aftermarket components I tried fit well with the Gen2 lower, I was disappointed that the design inhibited the use of several popular ambidextrous magazine catches.
Overall: * * * * 1/2
JMT’s Gen2 AR-15 Eighty-percent Lower is a rock-solid product that will appeal to experienced and novice builders alike. The included jig and nearly dummy-proof instructions allow completion of the lower in minimal time with basic tools. Gen2 has a few kinks to iron out, but that shouldn’t stop you from seriously considering this lower kit for your next eighty-percent build. The JMT Gen2 Eighty-percent Polymer Lower is an excellent option, available at a very fair price.