I recently ordered, and received, a JMT 80% lower. To date, I have purchased 10 Tactical Machine 80% lowers, both AR-15 and AR-10 versions. I have been very pleased with the TM lowers and jigs.

Having read through this entire thread, I am sincerely glad I didn't get one of the early JMT ones.

I have a couple of disclaimers to make before I submit my INITIAL RESPONSE to the JMT:

1) I have not milled the JMT unit yet,
2) I am not keenly interested in "plastic" receivers.

Upon opening the box, my first impression was "disappointment".
1) The unit does not look like the pictures on the website.
2) The jig is plastic. Right or wrong, I thought it was aluminum.
3) The jig is a "press fit". No screws. (Plastic explains that.)

Let me address each point.
1) The "look" is not TOO big a deal, EXCEPT that the receiver LOOKS way more "plastic" than "polymer" or more significantly, "composite". I have a reasonable amount of experience with "polymer" - as in the Remington "Nylon" .22 RF - and this is way more "plastic" than the Remington is. Is that "bad"? I suppose not, but I'm not a materials scientist. BUT, I don't "like" the LOOK. Second, along with the LOOK of "plastic", it SOUNDS like plastic when you flick it with your fingernail. That's NOT what "composite" sounds like, and it's REALLY not what "carbon fiber composite" sounds like.

2) The jig is seriously "plastic". It is the exact same stuff model airplanes and cars are made of. I cannot believe anyone can index this jig within 0.010" of flat or square, and don't tell me "you" can, because I checked it, and across its surfaces there are fluctuations of more than 0.020". That doesn't mean you can't drill a hole that is precisely positioned from one side to the other, but you are NOT going to get your setup to be "level" to better than 0.020" because the jig isn't any FLATTER than that. One could, and I will, put a flat plate across the face of the jig (thereby "averaging" the waves in the plastic jig's face), and that will "true" the face of the jig on average and I expect to be able to drill/ream true holes.

3) The fittings on the jig are tabs and holes. The jig COULD have been manufactured with threaded holes with nylon screws/bolts. The "right side" (piece of the jig) notches that receive the "left side" (piece of the jig) tabs are about the same level of precision you would see on a plastic model car or airplane. Absolutely no better. What this means is that "precision" can be a) no better than the slop in the tabs, and b) can NOT be consistent from one placement of the jig to the next.

I believe that I will be able to machine a working receiver from this unit and jig. I don't "like" - emotional, not "scientific" - the "plasticness" of it, as opposed to what I was expecting from the pictures and written description of it as "composite" and "carbon composite". It is certainly like NO OTHER "composite" or "carbon composite" I have felt. The proof is in the pudding as they say, and there are a couple of reports here in this thread by people that have actually SHOT this receiver, and their reports are positive. I expect to have the same outcome. Doesn't mean I "like" (emotionally) the "plastic-ness" of the thing, BUT, as I admitted up front, I'm not keenly interested in a plastic AR-15 receiver. Trouble is, I wasn't expecting a PLASTIC one. I offer these comments on "look" and "feel" not in criticism, but as "marketing" input for JMT. If people don't like how a product looks and feels, they don't part with their money unless performance significantly exceeds the ones they like the look and feel of.

One last concern regarding "plastic" that is not a concern with "carbon composite": Plastic gets VERY brittle in the cold. I live in Alaska. I gets cold here. I have some concern - cncern that I would not have with "carbon composite" - that this thing MIGHT break when it gets cold.

I applaud JMT for advancing technology. I expect their product will satisfy a certain part of the AR-15 community. I'm pretty sure this will be the last "plastic" 80% lower I purchase from ANYONE. Now, having held one in my hand, and the fact that aluminum TM lowers are at $59 'til the end of April, I think I'll remain an aluminum "kind of guy".

It occurs to me that the "draw" of an 80% plastic lower - besides the obvious - is that it is more reasonable to machine it with a drill press. While the aluminum 80% units are BEST milled with a mill, they CAN be finished (I am told) using only a drill press. I assume the plastic units make them more accessible to "the masses", and that is certainly a "good thing".

Paul

will these lowers handle a piston system? reason why i ask is that i saw a vieo on youtube where a guy milled a poly lower and put his rugger sr556 upper on it and the back of the lower snapped off after x amount of rounds.