Reinforcing a poly AR lower

WestKentucky · Oct 31, 2016

Poly lowers are known to fail at the takedown pin locations, especially the rear pin location. They simply flex a lot in that area and material fatigue finally results in a crack and break which essentially snaps the housing that accepts the buffer tube off from the rest of the lower.

It seems like minimizing flex in this location would significantly reduce the likelihood of a failure. There are companies now offering hybrid lowers to prevent this issue, but the problem remains with the many traditional poly lowers in the market already.

It also seems like there should be a simple solution to this problem by reinforcing the area between the pistol grip and the buffer tube attachment points. I am not aware of anything available commercially, but it seems an easy job with fiberglass or even something along the lines of a cast zinc or aluminum reinforcement bar to cover the problem area.

Has anybody tried anything to minimize flex and extend lifespan of their lower? I would like ideas as I am soon to build a poly lower and want to do some experimentation with it. If I can make a simple part that is a significant improvement I will gladly share photos and info.

If your not familiar with the breakage, just google "broken poly lower" and scroll through the pictures of this well documented problem.

easy · Oct 31, 2016

Best solution is to install an aluminum product.

LoonWulf · Nov 1, 2016

Some of the manufactures have started including cast in aluminum inserts. but as an after market solution I cant think of one. You could maybe do an aluminum or sheet steel saddle that connected both pin locations together along the side of the receiver, then went up and connected to the buffer tube some how....ive got a picture in my head, but no idea how to actually make it feasible.

boom boom · Nov 1, 2016

LoonWulf,
That is what I think might work as a retrofit--metals such as aluminum or steel , even thin metal, generally have superior shear and torsional strength (if I remember the terms correctly) than polymer. I think that you could do it as part of the spacer/sling attachment to the buffer tube/receiver interface. What I am thinking of is visualize something like an oversized arisaka or a late type m27 Finn Mosin bayonet band with long ears where the receiver pins go through the long ears. I am current rebuilding an old Finn Nagant barrelled receiver and noted the problems that the Finns went through trying to have a strong front band for bayonet work that would stand up to torsion. It turns out for the m27 that they put ears on the bayonet band to strengthen it and to resist torsion. Problem is that this would require welding most likely to attach the various pieces for individual manufacture.

A simpler construct might be similar to the Lee Enfield at the buttstock-forend interface where the wooden forend has a steel reinforcing bracket where reinforcing pins are inserted probably for a similar reason. That only require folding a flat thin gauge metal plate and drilling it.

easy · Nov 1, 2016

Sorry, my reference was to use an aluminum lower, either cast or forged. Problem solved.

LoonWulf · Nov 2, 2016

Course, and for 20 bucks extra i usually go for the aluminum. But if your doing an 80% with just jigs, Ive read multiple positives on the composite lowers, and a number of failures on the aluminum.

orionengnr · Nov 2, 2016

http://www.ar15.com/forums/t_3_4/67...IY_100s_of_ARs_cheaper__faster__gentler_.html

This one lasted 300 rounds. That is .25/rd...do the math.

Never tried this...and really, don't have a reason to (yet) but maybe cut a "strap" out of a Bud Light can using scissors and cast that into the plastic. Might take a few tries to get it right.
If that one "only" lasts 500 rounds.... find some aluminum that is a hair thicker.

Looks like these cost about $5 each to cast. Add the cost of one can of beer. That is inconsequential compared to the $75 worth of ammo to try and break it....
Looks like a fun project. And perhaps, something to keep in mind, for "some day".

Uh-oh, NSA is onto me now, and someone will be showing up at my doorstep momentarily...

Nice knowing you all.

Reinforcing a poly AR lower

WestKentucky

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easy

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LoonWulf

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boom boom

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orionengnr

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