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haewired

JMT 80% AR 308 Lower review (Pic Intensive)

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Ok everyone, here is my first review.  Just a disclaimer here and I want to stress that I'm nothing more than a enthusiast.  Never went to a armorer's course or tactical class, I just like to tinker and have a passion for firearms.  I think they're really cool machines and I like to share my experiences so others can learn from my mistakes.  This review will have two parts, part one will be about assembly fit and function.  Part two will be a short range report when I have a chance to visit the range.

This is my first 80% lower and it's polymer made by JMT the ".308 Equalizer".  Their selling point is their "own formula of hybrid polymers and fibers with additional reinforcements in high stress areas".  They also sell that their lowers are "engineered to Mil Spec tolerances" (quotes from their website).  The tools I used to mill out the cavity was a router and the Easy Jig Gen 2.  The upper I'm fitting this lower to is a JP side charger and I'll be putting out a review on that later.

The first thing I do with a lower is fitting parts, it's a habit I have and I like to see if things fit.  I don't know how it is for other 80% or even 100% polymer lowers but every part would not fit unless you forced it in with excessive force.  so every pocket needed to be custom fit with jeweler files, rotary tool, or 220 grit sandpaper.  It took a lot of patience with my calibrated eyeball and hands to get things right.  In the pictures below you'll see where I hand to make adjustments. 

               The largest adjustments I had to make was at both take down pins.  Fit of the pins were too tight and only one of the detents would engage when you pulled the pins out but when you fully engage the pins the detents will not click into place.  Maybe I'm being compulsive about this but I want to be sure the detent is fully engaged keeping the pins in place and not just friction.  So I used my dremel, steady hands, and nerves of steel to carefully remove as little material as possible.  I used a round file to open up the openings.

               The Bolt Catch slot and Magazine catch had to be slightly widened with a putty knife and sandpaper folded over the edge.  Constantly shaving and fitting parts was the key here.  The other issue here is when you install the Bolt Catch Pin, you're going to have a heck of a time getting it out if you need to replace it.  there isn't a lot of room to get that pin out. 

               The JP upper's take down anchor was binding in the lower so the lower cavity need to be sanded down.  The rear take down pin hole alignment was off as well, so I had to file down the mating surface of the lower so the upper and lower could align for the take down pin.  Luckily the pivot pin holes lined up just fine.

               Only other issues I had was the safety selector detent hole was a tad out of alignment (I'm being a bit over critical here).  It's either JMT's spec or my jig's alignment or a combination of both that caused this.  So when you look at the safety it looks a little crooked, and there was a over molded area on the rear of the receiver under the buffer tube I had to take down.  I didn't want the back plate to look crooked.  One other minor thing I discovered was a few small voids in the receiver itself I unveiled as I was milling out the cavity.  Probably from the molding process, but it was all very minor which is a good thing in my opinion. 

My next post will be the finished product

 

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Edited by haewired
afterthoughts

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here's the finished product.  After all that fitting, I polished off the sanded areas with finer sandpaper and stepped up the grit all the way to 1500.  Rough areas will catch dirt and grit faster than a smooth surface of course and it looks refined.  All in all I like the way this upper looks and the tight tolerances makes this a pretty solid feeling lower.  I don't know how it'll fit up with other uppers but that's the fun with .308 AR's as nothing is really "Milspec" in this realm, I'm quickly learning.  My main worry is if all this work was for nothing if it can't hold up to prolonged use and shock from the .308 round.  My advice to anyone who wants to try a polymer lower is be prepared to custom fit your lower to everything aside from the trigger and safety. 

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Couple things I forgot to mention is the lower has brass reinforced threads for the handle that is press fitted in and the engraving on the magwell was my addition to the receiver.  I had to carefully dig out the shavings from each letter with a dental pick so be careful if you do the same to a poly lower. 

Range report later on this same thread when I get a chance.

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Ok, I put 100 rounds through my rifle yesterday and took my rifle apart for cleaning and inspection.  What did I find?  My lower failed by cracking and stripping threads at the buffer tube.  You'll see in the pictures that the receiver extension ring threads are stripped out.  That explains why I had a misfire because the buffer retention pin broke loose at the end of the day.  Next you'll see a crack on the right side of the receiver starting from the top of the receiver above the take down pin, propagating down past the takedown pin and working its way to the handle.

I kinda expected this, but I had some hope that this polymer receiver would be the exception.  It was fun to build and I wanted the experience to make a functional 80% lower.  As far as I'm concerned it's a paper weight now.  I'm glad I didn't have a catastrophic failure while operating the rifle, and I'm happier to have learned about this experience and share it with you all.  I plan on posting another thread on this warning about this lower and any other poly lowers with a round count of over 100.  If any of you have a poly lower with a round count over 100, I highly suggest you inspect your lower.

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WOW....I figured the poly lowers would have been improved over the years. Guess not. that sucks. get you a metal one and put the parts from that one in it and call it done. sorry to see this. that was an awesome looking rifle too. but it can be again.

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I just received my "Raw" 80% lower from Juggernaut Tactical.  I have plans for that lower, and I guess I'll be writing a review on that one too since I haven't seen one just yet.

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Hope the new one works out for you, I was wondering how the polymer would hold up to the 308 recoil.

Edited by Belt Fed

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Was that magazine new? just an observation but in the third pic that mag follower  is mighty dirty for 100 rounds. looks like you had a ton of blowback. I clicked on the pic to enlarge it. That don't seem normal to me.

Edited by Belt Fed

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Magazine looks like mine after 20 rounds suppressed. Those cracks and stripped threads are not encouraging.

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Is the upper assembly over-gassed by chance? I had some serious violence in my hands with my first .308 build; before I figured out what was going on. The crack appears to have formed by the BCG action (unless you mortared the rifle at an odd angle). 

I've never built a polymer .308 lower; though, I've thought about it a few times... My earlier experiences steered me away for this particular reason. The .308 Win is a serious cartridge; just because a polymer lower works ok on an AR-15, doesn't mean it's well suited for the larger frame design. I still want to build one for myself, but I need another upper that runs flawlessly before I consider it an option.

Glad to see someone else giving it a try; and more glad that you didn't hurt yourself in the process...

Edited by Lane
spelling is hard

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3 hours ago, Belt Fed said:

Did the lower come with any kind of warranty or guarantee?

Yes, there’s a limited warranty as long as it was used under ”Normal Use” and as long as the damage isn’t adjacent to a milled or drilled area.  To be honest, I don’t want to trouble myself with going through the paperwork process, and building process all over again for it to fail.  This was a fun experiment at best, and if it held up to the punishment of the 308, then awesome.  I’ve never built a 80% lower before and now I’m graduating to a aluminum lower.  After this case I don’t think I’d ever consider a .308 Polymer lower for serious use.

1 hour ago, Belt Fed said:

Was that magazine new? just an observation but in the third pic that mag follower  is mighty dirty for 100 rounds. looks like you had a ton of blowback. I clicked on the pic to enlarge it. That don't seem normal to me.

Magazine is brand new.  I shoot suppressed 98% of the time as my family’s property has other houses nearby.  I like to be a good neighbor = ].  From my experience shooting AR15’s without a adjustable gas block, that carbon you see is completely normal in my opinion.  

47 minutes ago, jtallen83 said:

Magazine looks like mine after 20 rounds suppressed. Those cracks and stripped threads are not encouraging.

I concur sir!

11 minutes ago, Lane said:

Is the upper assembly over-gassed by chance? I had some serious violence in my hands with my first .308 build; before I figured out what was going on. The crack appears to have formed by the BCG action (unless you mortared the rifle at an odd angle). 

I've never built a polymer .308 lower; though, I've thought about it a few times... My earlier experiences steered me away for this particular reason. The .308 Win is a serious cartridge; just because a polymer lower works ok on an AR-15, doesn't mean it's well suited for the larger frame design. I still want to build one for myself, but I need another upper that runs flawlessly before I consider it an option.

Glad to see someone else giving it a try; and more glad that you didn't hurt yourself in the process...

I have a JP Rifles adjustable gas block installed and it’s optimized for unsuppressed operation.  Shooting suppressed will cause more back pressure but with a adjustable gas block it mitigates a lot of the gas that vents through the back of the receiver to the shooters face.  It also prevents the early wear and tear of the rifle and associated parts.  Thank goodness for adjustable gas blocks!  As far as the action is concerned, I didn’t feel any violence or jarring motions when firing my rifle.  The action felt smooth without any kind of a smacking feeling at all.  Now I did have a few rapid fire events to check for functionality.  I don’t think this lower or any polymer lower will be handle this course of fire.  I don’t know if JMT Enterprises would consider that “Normal Operation”.  

After this experience, I would say that if you want to try to build a polymer lower and you plan to shoot 20 rounds a year it’s fine.  I however would highly recommend a complete stripping of the lower and inspection after every 60 to 80 rounds.  I can’t speak for AR15 Polymer lowers though.

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Here’s a pic of the buffer retainer detent.  I thought this was due to the fact that the receiver extension barely engaged the detent.  Now instead I believe that lower polymer had enough flexion when fired to cause tolerances to open up damaging this part.  This caused a misfire and when I took apart the upper from the lower it just fell out and the buffer assembly popped out.

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I shouldn't act totally shocked by these results; but I'm still really curious about the materials involved, and that crack. I don't see a any fibers in the areas you milled; or near where the crack formed. Perhaps they only add fibers in areas that matter, which is why the crack happened there? I've seen pictures of polymer ar-15s that broke off the whole buffer tube mount; but that didn't happen to you. That may be reinforced; but the rear takedown area is not? 

Can't even begin to defend them on the buffer tube threads though; that's a secondary issue.

While it may be a waste of time and paperwork; I'd bet they would replace it at the very least, especially so if you only had 100 rounds through it. The fact that you had an adjustable gas block, and it was adjusted is pretty sound proof that it was "normal operation". At the very least; letting them see the damage will hopefully save someone else the trouble in the future (if they actually fix the problem once they see it). I want to believe; it's in their best interest to do so...

And again; very glad you didn't get hurt in the process.

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I'm like Haewired on the warranty issue, if they would give him his money back then that would be great. but to send another lower to put time in and it fails is a waste.

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On 5/17/2019 at 9:00 AM, mineralman55 said:

Looking forward to the range report. Do you have any pics of the jig setup and routing? I'd like to see those as well.

As requested

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Holt SHIIT!   That jig looks like it would cost more than a LaRue rifle!

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