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gatordev
1 April 2017, 14:22
Posting this as part hypothetical, part understanding an actual issue, and looking for some thoughts. This is specific to the AR-15 (NOT the AR-308) and I'm not actually trying to prove what I'm asking is true, instead I'm trying to understand if this is even possible. That said, here's the background, followed by the question:

Weapon:

-12.5" not-overly-gassed carbine-length gas system by a quality, known manufacturer
-Quality BCG by same known manufacturer with less than 3K rounds
-Spike's T2 (or whatever it's called) buffer and proper spring with 3K rounds
-SF Legacy muzzle brake (so maybe just a slight bit more back pressure than a FH)
- Ammo is Federal M193

Scenario:

Weapon runs like a sewing machine. No issues for almost 3K rounds.

Question:

Would changing out the buffer to a carbine weight buffer (and brand new LMT action spring) cause a condition that would result in sheared lugs on the bolt within less than 15 rounds of shooting?

Discussion:

I understand bolt bounce is a thing (though I can't prove it was or was not happening with the carbine buffer), but even with it, would there be a scenario where the lighter buffer would cause lugs to shear? I wouldn't think so, but I'm open to learning new stuff.

UWone77
1 April 2017, 16:28
I can't imagine that would shear the lugs off within 15 rounds.

Before I knew any better, I ran a carbine buffer on a 10.5" Pistol/turned SBR later for a few thousand rounds. It ran fine, but I definitely could have tuned it better.

FortTom
1 April 2017, 17:25
Wow, sounds like one of those "point is space" questions from Calculus. My gut instinct would be no, but changing springs every 5K or so wouldn't hurt. There's a hell of a lot of things going on there, physics wise, strength of materials, hell you could cram half of engineering school in to solving that question. I've changed springs and buffers many times to "tune" a rifle that wasn't cycling "just right" and have yet to suffer at the hand of the frightening physics book. I think that's a combination of metallurgy, physics, strength of materials, mechanics, and a number of other specialties wrapped up in one question. "Modern" physics grew out of minds of the great philosophers. Galileo is often refereed to as the "father of physics", and if I had to guess, he'd just tell you - 'Fkit" just go shoot it and find out.[:D]

Then let us know how it worked out for you so the rest of us don't screw our stuff up.[:D]

P.S. My real guess would be that your bolt had a stress fracture or something along those lines and was ready to give up the ghost anyway. The fact that you changed a buffer and spring, I would guess, was just happenstance. The fact that it was a brand that makes quality parts is probably moot in this instance. Even the best of mechanical parts, especially those that go through a lot of dynamic stress, break once in a while. Even the best Quality Control/SPC program a business has can't guarantee 100% a zero chance of failure, as I'm sure you know.

FT

Stone
1 April 2017, 17:40
Sounds like the lugs were going to shear off regardless of what you swapped out. Just freak timing, doesn't mean you should start connecting dots. Bolts are consumable parts. The real question is will this "quality bolt manufacture" warranty it. [pop]

gatordev
1 April 2017, 17:58
Good words about how this doesn't make physical sense...

Pretty much what I was thinking, but figured I'd see if I was out in left field.


Sounds like the lugs were going to shear off regardless of what you swapped out. Just freak timing, doesn't mean you should start connecting dots. Bolts are consumable parts. The real question is will this "quality bolt manufacture" warranty it. [pop]

I agree that bolts are consumables. By that same token, I don't expect anything from the "quality manufacturer." But if they provide anything, that's great, too. For now, I need to take a few pictures and give it time since it's the weekend.

FortTom
1 April 2017, 20:42
Pretty much what I was thinking, but figured I'd see if I was out in left field.



I agree that bolts are consumables. By that same token, I don't expect anything from the "quality manufacturer." But if they provide anything, that's great, too. For now, I need to take a few pictures and give it time since it's the weekend.

Out of curiosity, how many lugs sheared off? Over the years I've only seen a couple of bolt's "eat" themselves, in semi-auto weapons, but they were mostly gun show bargain bin stuff thrown together like a jigsaw puzzle. And for myself, I had one that never would cycle properly that, too, was from a brand name and a sponsor here, that pissed me off so bad that I trashed the damn thing, rather than even talk about a warranty. (that was for a LR-10 though). I also use M16 bolts thinking they might be more durable, but not actually knowing that.

FT.[:D]

UWone77
1 April 2017, 20:51
Also reminds of several bad batches of bolts Rainier got during the post Sandy Hook Panic.

Batch #1 improper heat treatments
Batch #2 Ordered 9310, got 8620 bolts

All had massive returns for sheared off lugs

gatordev
2 April 2017, 08:45
Out of curiosity, how many lugs sheared off?

FT.[:D]

Yes.

But seriously, I don't know if they all went at the same time. I had two malfunctions back to back and it's possible a lug may have sheared during the first malfunction. After the second one, the rest of them fell out of the gun when I was clearing it.


Also reminds of several bad batches of bolts Rainier got during the post Sandy Hook Panic.



I can't prove it yet, but I'm thinking this might be a post-Sandy Hook bolt. I haven't been able to find a receipt yet, but given when I know I sourced the parts for this gun, the timing matches. Honestly, that's the biggest reason I want to give feedback to the company to see if it's part of a bigger issue.

Joelski
2 April 2017, 12:05
Failure at 3k rounds seems awful fluky. I doubt it'll ever happen to you again, unless you have luck like mine. If that's the case, I sure as hell wouldn't fly helo's any more! [:D]


What does shearing lugs do to the barrel extension? Haven't experienced it yet (although I anticipate what I can), but what I mean is, a lot of the industry makes a big deal about their bolts being machined from Carpenter 158, or 9310 steel, but haven't heard much about the metal used in barrel extensions. One would think that the force required to shear bolt lugs over time would do something similar to the barrel extension's mating surfaces, no? Each time that bolt slams home, something's gotta absorb the energy. Is it just injerently a low-wear item that outlives rifling and gets canned? I doubt there's much data outside of the milltary due to round count-related failures.