In the early part of this year I posted a thread asking if anyone had built one of those 80% Poly AR lowers that had just hit the market. We talked a little about them, but nobody had built one, so I decided to give it a shot.
At the time there were three main companies putting these things out. Polymer 80, James Madison Tactical, and EP armory. Polymer 80 and JMT sell a Jig to finish the lower, while EP advertised that you didn't need one. After staring at all the photo's I could find I thought that EP's lower looked chinsy, and I wanted a Jig. That decision turned out to keep my name out of the subsequent BATFE saga when EP and Aries got raided.
I bought one each of Polymer 80's and JMT's products. At the time JMT was a cheaper lower, but sold the Jig seperatly. Now they appear to include it in the package. Either way both of them were right at $100 to get a lower and Jig. Polymer 80 sent me some drill bits and an end mill which was nice.
Polymer 80:
JMT:
P80's jig needs to be screwed together with those little wood screws and the Jig itself is flimsier. If you clamp it too hard it'll flex. JMT's snaps together VERY tightly, with no flex, and no play. You actually will need to pry that little piece on top off with a knife if you get ahead of yourself and snap it on for a picture.
I have access to a mill at work, but I thought the whole point of this project was to open 80% building to folks without expensive equipment. So I did all of it on my Harbor Freight cross slide vise and my smaller drill press, a Chinese one I got at Home Depot about 10 years ago.
Tools needed:
P80: I printed out the instructions and started with this one. First cut was at the rear of the fire control pocket where the takedown pin goes through. The jig's lack of precision became apparent at this point.
No biggie I just picked a side and got to milling. After the first cut you actually mill off the top part of the jig (They both tell you it's a one time use Jig) to set up for the fire control pocket. then it's just milling down to finished depth. I plunged the end mill about 1/16" per cut. Probably could have gone more, but I didn't want to load up the drill press too much. Fire control pocket milled:
Then you have to flip the lower on it's side and drill three holes. One each for the hammer and trigger pins and one for the safety. What problems other folks had on the internet came at this step. It's important to make sure these holes are square, even and in the right place. Instructions vary, but I chose to drill straight through both sides in one operation, rather then drilling one side and flipping, re-leveling, and drilling the other.
Squareing to the drill bit:
And Drilling:
This is where the first disaster struck. Remember how I said that the Jig flexed if you clamped it too hard? Well in trying to keep the jig from flexing away from the lower I didn't clamp it hard enough and while drilling the safety hole the bit grabbed the jig on the back side and the lower climbed up the bit. This made one side of the safety hole slightly out of round.
Messed up safety hole:
At this point I cleaned up the edges with a sharp knife and tried to assemble the lower. No Go, the trigger wouldn't fit. FCP was too narrow. I screwed it back into the jig, and rechucked the end mill. I took a little off each side until the trigger dropped in cleanly. I ended up having to open the pocket up a little more then .0035" wider then the Jig. It was a PSA trigger, and everything has tolerances. I recomend test fitting the trigger and hammer before taking the lower out of the drill press after milling. Just to be sure. After I got the trigger and hammer in I installed the safety with baited breath. It has a little wiggle on the left side but functions fine, stops the trigger when engaged, and the lever flange completely covers the hole. So no harm, no foul. P80 also has a reinforcing rib on the bolt catch. This means if you used a roll pin you'd never get it out. So they include a loose fitting pivot pin and a little set screw to retain it. This is hands down the worst part of this design. The bolt catch and pin go in, but that stupid screw.... It won't thread itself into the polymer (ploymer's too tough) the stupid hole is too small to just drive a normal roll pin into, and to top it off because of it's location on the lower, you can only turn it about 1/3 a turn at a time with the stupid, tiny allen wrench. I ended up getting the right tap from my set, tapping the hole, then threading it in. But it was a PITA, and still isn't straight if you look too close. Pulling it out to change the bolt catch is a pipe dream. No picture here because I was too pissed. Everything else went together like a standard lower.
JMT next:
There were some very obvious differences right away. I already mentioned the jig is tighter and stiffer. Whatever polymer JMT uses machines much nicer then P80's. Cleaner lines at the edges, and it's stiffer as well. JMT obviously assumes you have some machining experience as well. Where P80's instructions say things like "mover the bit slowly in this pattern [picture] while lowering it a little each pass.", JMT gives you the dimensions of the void and says "Mill this void". P80 gives it's measurements from the top of the jig. JMT gives you the dimensions of the void and says "Mill this". If you know what you're doing it's no problem, but if you've never done this before it could be intimidating.
Here's my set-up to monitor my depth of cut:
Cutting:
Finished Fire Control Pocket:
You can see the walls of the void are cleaner even before a clean up.
Flipped the jig and drilled the three holes. Once again JMT's Jig was superior. It clamped tighter and didn't move. Having learned from the first time I test fit the hammer and trigger before I took it out, and I had to open this one up a little past the Jig as well. I think they make the jigs a little tight in case you overshoot. Assembly of this lower was uneventful.
Finished guns:
Pre shooting thoughts:
I Like JMT's product better. It feels more solid, machines better, comes with a better jig, and assembles easier. On the downside the instructions require more knowledge, you'd need to provide your own bits and end-mill so that would add some money to the project, due to some reinforcement on the buffer tower, you need a round of ammo, or a tool of some kind to pop the take down pin. Upper to lower fit is tight, with no perceptible movement when assembled.
P80's product works fine. (except for that little screw). It's instructions are easier to understand for a novice, it won't require fancy measuring, and all the cutters you need are included. One stop shopping.
Shooting thoughts:
Not much to say here. I have put 1000 rounds of 5.56 through the P80 lowered pistol, and about 600rds of .300blk through the JMT. They feel like shooting an aluminum lower. No failures on either gun, no signs of wear or failure on the poly lowers. Working fine. I put anti rotation pins in the .300, but I'm not sure they are needed. Cheap insurance if you are worried about wear on the trigger pin holes. But both products appear to make fine firearms that work. I haven't
tried to break them but I have tossed them around in the trunk of a car, in a backpack while camping, and a little run and gun type training. The pistol has become my go-to long gun for road trips. Reliable, accurate and compact.
Closing thoughts:
Building and smithing on my guns is part of the hobby for me. I like making them. So to me building one is worth the small extra expense. ($100 purchase vs. $49.99 lower + $25 transfer) I'm already collecting parts for a 9mm AR pistol and it'll probably get a JMT lower in black. Both companies have put out a Gen 2 product since I bought mine, and both look good. I will go with JMT mostly because I like their polymer better. Cleaner machining and stiffer finished product. None of these need to be babyied safe queens. I think that these are plenty tough enough to be considered weapons, even if I might choose a metal lower for a war zone.
I like them. and it's cool to have to AR's with no serial numbers at the range. Some folks get all butt hurt on that one.
