Polymer 80% lower failure AR 308

[haewired]

Hey all, just wanted to share my experience with a late model Polymer 308 lower.  When I started this project, I was looking something in the 80% realm which is something I've always wanted to try.  I was also looking for something that looked cool and in large caliber.  Since Polymer construction has been around for a while now I thought I'd take a leap of faith that the technology and experience is up to the level where it could be used in applications like this.  So I did some on line research and came up fruitless so I thought I should give it a try anyway.  My rifle has a 18" 1/10 Criterion barrel with a mid length gas system.  I'm using a heavy JP rifles Heavy BCG and a Extra heavy 308 buffer.  I think the combined weight is 2 lbs.  I'm also using a JP Rifles adjustable gas block.  My rifle gas block is optimized for un-suppressed ammunition.

Constructing the lower wasn't easy as there no such thing as "Milspec" in the 308 AR world (as far as I've researched).  So I had to pretty much custom fit my lower to the parts I have.  After my labor of love (So to speak) I field tested my rifle and went through a total of 100 rounds of assorted brands of 308.  My rifle pretty much operated well with the exception of me lubricating everything but the BCG causing a failure of return to battery, and the Buffer weight detent leaving it's rightful place in the receiver extension ring causing a misfire at the 80th round.  It was pretty much a good day.

The next day I stripped and inspected my lower and found a couple critical issues.  First thing I found was the Receiver Extension ring threads were stripped, isolated to the lower portion of the ring.  I think that was the culprit for the Buffer detent leaving it's spot causing the misfire I had.  Second, I found a crack right above the Rear Take Down pin that propagated down working toward the handle on the right side.  When you look at the pictures, you'll see an unusual amount of carbon build up after 100 rounds.  I shoot suppressed 98% of the time to be a good neighbor.  So this build up is something I'm familiar with  AR15's.  The other thing I'd like to mention is the rifle's action felt very smooth.  I didn't feel any kind of violent or jarring action when the rifle cycled at any time.  I've shot other AR 308's in the past and this rifle felt smoother thanks to the wealth of knowledge from this forum.  So in my personal opinion my rifle was working fine, so you'll have to pretty much take my word for it and trust me. 

I'm including a you tube video clip of my field test so you can get an idea of how the rifle was operating and the pace I was putting it through. 

 

If you're curious my lower is made by James Madison Tactical.  There is a "Limited Lifetime Warranty" but there are a few caveats involved.  I won't get into it here but ultimately I don't want to go through the trouble of custom fitting my parts all over again, and spending the time & money to test it all over again to most likely get the same results.  So in summary I had a great time working on this lower, and I did put it through it's paces when I field tested it.  Looking at then end result I don't think I would ever use a Polymer lower again.  If you ever plan on trying one out or if you already have one, I would highly recommend stripping it down and giving it a through inspection every 50 rounds and be very careful.  Honestly I wouldn't go near one ever again.  I hope my personal experience will help some of you in some way.  Be safe out there!

[MrPaul]

That's a really nice rifle. I wonder if the back pressure from shooting with the suppressor had anything to do with the crack on your rear takedown pin hole.  I run a polymer 80 lower on 1 of my guns that has few thousand rounds through it and haven't had any cracks.  The stripping on the buffer tube threads was an issue on mine so I just figured the buffer tube was permanently attached once I got it back on straight.

[haewired]

Thanks, the gas system was optimized for unsuppressed fire, so there's not doubt that my rifle was over gassed when firing suppressed.  Even though I shoot mostly suppressed, I still want releability when shooting unsuppressed.  Maybe that had something to do with the stress crack.  How did you secure the buffer tube to the receiver with stripped threads?  I'd be worried about that receiver extension, BCG, and weight flying off in a different direction.  For me even if there were no cracks on the receiver, I'd ground the receiver if the buffer tube threads were stripped in any way.

[Jaystactical]

In the AR platform you have a bit of micro wobble. In the .308 with a polymer lower it is like firing a 155 Howitzer on top of a matchbox car. That micro movement of a millimeter back and forth per round wears those rear take down pin holes with the bolt and buffer rocking forward and backwards. Secondly the heat created during milling changes the density of the polymer. The buffer tube threading is maybe just installing the buffer or could also be from when the spring is compressed it is creating a bind point that causes the weakest point to flex. The aluminum tube will not flex, the soft polymer will.