Polymer AR lower catastrophic failure.

Glad you remained intact even after it didn’t.

I would say at 6500 rounds you got your money’s worth from that lower.

Safe shooting!

Classic location. The takedown pin hole really shows as a weak point every time that I see a pic of a failed poly. I'm glad to hear it was quite uneventful.
Did you find all of the little pieces like detent, detent spring, takedown pin?
Was there any damage or indication of impact of the bcg on the misaligned parts? Full throttle on gas, or did you have an adjustable block turned down?

I ask because I have a poly that I am intending to replace, but haven't yet. It seems that they could make a poly with thicker web in this critical area, even if they did so by adding material on the external flats. Would be cheaper and easier than adding a skeleton to the mold before injecting.

yup that's where most fail. put it on a plack as a reminder .

I’m glad you’re ok and the only thing was a wobbly receiver.

I don't understand why people mess around with polymer lowers when real metal ones are so reasonably priced.

My father in law has a bushmaster carbon 15. That one seams reasonably reinforced. The only one I would buy personally is the gwacs one.

AlexanderA said:

I don't understand why people mess around with polymer lowers when real metal ones are so reasonably priced.

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Key word is 80% and very little paper trail. Polymer is easier to work with than Aluminum for a lot of people. Otherwise, they are a bit cheaper and lighter. One polymer lower is literally poured into molds and requires little if no filing afterwards. As the post above notes, it is strong enough to last thousands of rounds. Easy peasy.

Second, a lot of folks have familiarity with polymer now due to Glocks, etc. The polymer Glock 80% knockoffs have me even thinking about building one as a lark. T

Expect the anti-gunners to start cracking down on "ghost guns" as their latest crusade which will be self defeating as nothing so much makes someone want to buy something as forbidden fruit.

I have a couple of Cav plastic lowers one that I even beat up with 458 socom for awhile. They don’t use a thread in buffer tube or separate grip though so are not as fragile in the area where yours broke.

someguy2800 said:

My father in law has a bushmaster carbon 15. That one seams reasonably reinforced. The only one I would buy personally is the gwacs one.

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The Carbon 15 isn't exactly a great rifle but I trust them a lot more than I would trust one of the 80% polymer lowers. I don't know about the 80%'ers but I do believe the Carbon 15 is fiber reinforced, which should be less prone to breaking. My dad used to have one and between the 2 of us I'd estimate it easily had 5-6k rounds through it without any issues. It seemed to like the BCG to be dripping wet if we wanted to put more than 3 mags through it in a range session, but CLP is cheap enough.

KansasSasquatch said:

The Carbon 15 isn't exactly a great rifle but I trust them a lot more than I would trust one of the 80% polymer lowers. I don't know about the 80%'ers but I do believe the Carbon 15 is fiber reinforced, which should be less prone to breaking. My dad used to have one and between the 2 of us I'd estimate it easily had 5-6k rounds through it without any issues. It seemed to like the BCG to be dripping wet if we wanted to put more than 3 mags through it in a range session, but CLP is cheap enough.

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Pure nylon is very soft and flexible, like a cool whip tub, so all of the polymer lowers and polymer pistols are going to have a high percentage of filler to get the flexural modules to where it needs to be, either chopped carbon, glass, or both. Probably like 40% of the material is actually filler. I believe the bushmaster ones are carbon filled, hence the name. It’s alot lighter than glass filled and let’s the manufacture claim its “carbon fiber polymer”.

I work in the plastics industry and the things you can do with the right polymers and good design for manufacturing are impressive. A Glock is a better polymer pistol than if you tried to make it out of aluminum because it was designed with polymer in mind, but you can’t take a design that was meant to be metal, like an AR15 or 1911 and just directly reproduce it in polymer without redesigning it first.

jmorris said:

I have a couple of Cav plastic lowers one that I even beat up with 458 socom for awhile. They don’t use a thread in buffer tube or separate grip though so are not as fragile in the area where yours broke.

View attachment 815388

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I bought one of those when they were on sale for $89. I wish I had bought a dozen. I put a 16” pencil barrel upper on it and it has become easily my favorite rifle.

jmorris said:

I have a couple of Cav plastic lowers one that I even beat up with 458 socom for awhile. They don’t use a thread in buffer tube or separate grip though so are not as fragile in the area where yours broke.

View attachment 815388

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The Cav/GWACS poly lower has more material in that area, too.

View attachment 815524

I am inches away from getting a couple 80% polymer lowers. would they hold up to 6 grendel or 6.8?

I don't have any experience with these polymer lowers but I do have a Windham weapon Carbon Fiber Ar and it was designed with significantly more material in critical areas compared to my aluminum lowers . Does anyone here have experience with Windham's offering when it comes to durability ? It feels and looks like quality .

I own one of the Bushmaster Carbon 15"s ( R97S ? ) ... super LtWt... but sadly not anywhere near as strong as an 7075 lower.

And I am just going to throw this out there... what are the odds the OP's could be glued back together>

I realize it would be easier to just buy a 7075 lower... but for the sake of argument ?

Plastic is not very accepting of glues and epoxies. It will never be as strong as it was before, which was apparently inadequate to begin with.

Gun buyback $$$$!!!!

jeeptim said:

It was a cheap 6/7 year old plastic 80% No reason other then it was a
P.O.S. functioned well for way longer then I ever thought.
Also had no signs of letting loose.
So if you gots one expect it.

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By chance do you have a pic of the inside of the receiver? It seems many of the poly failures with similar location have been the 80%'s. Not sure if its a milling/heating/stress issue or sidewalls cut down too thin, or possibly they are made with different polymers than complete factory lowers?

Should have gotten one of these.
https://www.tnarmsco.com/liberator-80-receiver-blank-w-jig/

Aluminum budget lowers such as Andersons and PSAs go for $30-50 on sale. There is merit in the comment that polymer is easier to cut at home. Still for longevity, good anodized aluminum has it in spades over polymer.