During an event yesterday, a friend ran into a dead trigger that could not be revived with immediate action. He transitioned to a sidearm, and completed the drill. Afterward, in breaking down the rifle, nothing about the trigger was immediately evident, but a sheared bolt lug was identified. We figured the detached lug ended up tangled in the trigger mechanism, causing the dead trigger, but then fell free, in opening the weapon, as we couldn't find it, once we knew what to look for. Long story short, a BCM bolt broke. Not anything torturous about it's life. Maybe 3k rounds on it. Colt model 933 with a SiCo Specwar, shooting factory 5.56 ammo. I guess it was just a lemon, which can happen to any manufacturer, but I was under the impression the HP and MPI testing exists to ensure this kind of failure, under normal conditions, is an impossibility.

Duty cycles are shortened with suppressed use especially on short barreled guns but yeah, that's a bit short. What type steel is the bolt? 158 or other?

That's why having a spare BCG is a good idea no matter who makes it.

Good time to get a SOLGW BCG.
They'll replace broken or worn out stuff.

When it happened, I told him I thought I had a bolt in my gear. I knew I had intended to start carrying one. After fishing around and coming up with nothing, I figured it never did make it's way in there. Sure as shit, it was laying on it's own in a drawer, conspicuously where I wouldn't forget about it LOL. It's in the gear now. And it's a BCM! AAAHHHH! I have an LMT enhanced bolt in my rifle, and I don't expect it to break, but like you said: good to have.

I am not sure what BCM makes their bolts out of. I think the failure was inappropriate, regardless of barrel length and silencer use.

This message has been edited. Last edited by: KSGM,

quote:

Originally posted by powermad:
That's why having a spare BCG is a good idea no matter who makes it.

And if nothing else- everything minus the carrier- bolt, cam pin, firing pin, firing pin retaining pin. These parts are easily carried and take up little space.

As a matter of fact, MagPul makes or did make an insert for their MOE grip which housed the bolt and firing pin, but I wouldn't recommend it. It's not very secure at all.

Keep in mind that a sheared lug can also be indicative of another issue. An overcharge of powder or bullet setback/COAL issue. Failure mode of a sheared lug is a safe failure.

But concur. It's disappointing; I would contact BCM and see if they would replace or want to inspect it

They are C158.
Pretty sure they are not batch tested.
Should be able to see if it has been pressure tested by the bolt face when new.

I would check barrel extension alignment with the upper before running a new bolt through it.
If that checks good I would just chalk it up to shit happens and motor on.

Yep, just checked...158 and all goods sold "as-is" no warranty

I did consider the ammo being to blame. Over the past six months, I experienced an anomalous baffle strike, another friend had a squib, and now potentially this bolt breakage. All squared away guns with factory ammo. It sucks to think that factory ammo potentially has this kind of inconsistency.

I did encourage him to contact BCM.

This particular bolt was purchased years ago, and is marked BCM, HP and MPI. It may not be representative of their current output. Shit does indeed happen; we are certainly motoring.

Best to contact BCM, keep the details to-the-point, include pictures. They may be keeping some records or, noticing a pattern of faulty metallurgy for a specific order that was produced. A good customer works with the brands they like, BCM has always been solid with me and I'm apt to give them the benefit of the doubt, along with understanding that lemons do get through the process.

I sorta missed the Colt 933 part and suppressed.
3,000 rounds is probably a good run for that setup.

I would look at a BRT gas drive that's sized for the action and suppressed use.

I use BRT tubes, and am actually considering getting him one for his upcoming birthday. His job has him busy enough that he isn't quite up to speed on current silencer-friendly options.

I still think that is a pretty low round count, even with the short barrel and silencer. Have you seen other examples of bolt failure on similar builds with similar round counts?

Good discussion recently on breaking bolts at m4c
https://www.m4carbine.net/show...-Really-Breaks-Bolts

There is nothing in that discussion that led me to believe the failure of this bolt was normal. I would never dispute that shorter barrels, especially when suppressed, are harder on components, but this failure still seems ammo-related, or due to a bad bolt.

I take military weapon anecdotes with a grain of salt. I saw first hand, an entire Brigade's worth of brand new FN M4A1s burn through gas rings like it was their job. The unit was able to salvage the BCGs from their outgoing Colt rifles, and the problem was solved. I think military arsenals see a higher frequency of bad components than the commercial market does.

One thing I will say is the round count on his gun is unknown. The barrel isn't toast or anything, but the gas port may be eroded, which would presumably make the stress that much worse.

It seems that even with military use broken bolts were not common.

A regularly gassed SBR with a can and hot ammo can make for short component life and is a compromise.
3,000 rounds may not seem a lot, but under the right conditions may be all that it can handle.
I would think that a bad bolt would have checked out long before 3,000 rounds, but I've never burned out a barrel let alone had a broken bolt.

My take away from that is to use a quality BCG and have a spare for the rare chance you may need one.

I'd appreciate a word from Fritz. He's the dude that goes through barrels, so surely he's broke a bolt.

I have never broken a lug on a bolt. I have retired 2 bolts. The retired parkerized bolt was showing quite noticeable wear on one lug when I pulled a barrel, so that bolt is in a box of parts somewhere. The retired NiB bolt shows minimal wear on lugs, and probably could still be used for quite awhile without issues. Probably could use a new extractor spring, however.

In one steel match a guy broke a lug during a field stage. Jammed his rifle, cleared the jam, then kept on shooting. He didn't know about the sheered lug until after the match, when he pulled the BCG for a quick inspection. I don't recall any brand details. I know he shot fairly stout hand loads, using a suppressor. His bolt and BCG looked pretty dry and dirty, and he stated the rifle had quite a few rounds on it.

Not that it solves any mysteries, but Vickers has a video with BCM talking about their quality control that I enjoyed. I didn't comb through the video to look at what they say about the bolts, but I recall the BCG being a heavy amount of focus.

https://www.youtube.com/watch?v=BaFOZAwspYI

The square lugs of the bolt create stress risers at the base of the lug, just from the nature of having sharp corners. Uneven loading can concentrate pressure on a single lug, and/or something as simple as a little bit of breakout from the tool that cut the part might be enough to weaken the lug and induce a failure - immediate or over time.

Magnetic particle inspection is looking for voids or other discontinuities in the profile of the steel within the bolt, a material defect like incomplete grain formation is usually detectable if issues occurred during heat treat, or if the billet being used had a defect such as a void/occlusion, contamination, etc. It will NOT find a minor machining error or defect.

High pressure test is checking the strength of the part and is typically only a single round, which is manually extracted/ejected to prevent wear from cycling the overpressure round. Again, it will NOT find a minor error that will only show itself after wear and fatigue have set in, it is looking for structural weaknesses and issues in the formation of the steel.

Was it the bolt's fault/a bad bolt? Possibly. Hard to tell without an inspection and gauging of all the parts associated to ensure there was no misalignment, odd wear, or other factor that may have differentially loaded the bolt's lugs or otherwise contributed to the failure.

As a general note - the lugs on either side of the extractor are typically the first to go as the missing lug for the extractor isn't sharing the load. If your failed lug isn't one of those two and there isn't wear or some other issue present in the fit between the parts it may more likely be the bolt's fault.

I haven't broken any bolts in my guns. The 5 or so I've seen in person were all overgassed, underbuffered, failures waiting to happen - broken at the cam pin bore, sheared lugs with evidence of an off-kilter barrel extension (all the lugs had deep offset wear corresponding to where the barrel extension lugs were rubbing), cracks formed in lugs with evidence of late unlocking in the recoil stroke (all lugs had deep smeared wear on the rear faces). Round counts were mostly unreliable or unknown. I've seen a few dozen in internet groups and from instructors - very infrequently a "good" brand factory gun, more commonly a homebrew with interference issues and bad wear or budget brand with short barrel, underbuffered, overgassed, etc.

KSMG I don't actually follow the theme on this thread. I consider every part of the AR operating system consumable (barrel (though maybe not as often as fritz), bolt, springs, trigger group parts, mag release (I break those all the time) etc.). HP and MPI testing don't make parts somehow super parts good forever. They still wear and will ultimately fail. The guys at BLV who I think have a lot of good info say they get 20K rounds out of a bolt and that's probably -on average- service less severe than a suppressed 933. So 'maybe' 3K rounds is the bottom of the bell curve. Doesn't sound like great service life but doesn't sound defective either. I've had 3 bolt failures in the course of my shooting. And I've retired a bunch more bolt parts because I didn't like some aspect of the wear pattern as a precaution. These all being top of the line parts from people who know how to make these things. I searched for some mil spec info on bolt life testing but didn't find anything of note sadly. But I'm not getting too excited about this failure myself.

I appreciate the replies, fellas. I don't know that there was an intended theme, when I started the thread, hrcjon. It was the first bolt failure I had seen firsthand, and, knowing the shooter and the history of the gun, I was surprised by it. In reading responses here, and elsewhere, it seems there is no real consistency, in regards to bolt failure experiences. The majority seems to have never experienced one, even having fired many thousands of rounds. Yet plenty of folks seem to think a failure like that is acceptable under certain conditions: in this case, a silencer on a 11.5" barrel. Considering what I have read here and elsewhere, I am inclined to think this failure was inspired by a perfect storm. Perhaps a bolt with a machining inconsistency, combined with a mild rifle stressor, beyond merely being a shorty with a can. Overall, I guess it really doesn't friggen matter. I have no beef with the bolt manufacturer, and neither does the rifle's owner. I am definitely going to pass on what I have learned here, to the shooter, so that he can inspect components for uneven wear patterns.

It's just odd how info on this stuff is so erratic. Fritz shoots a lot, with a can, yet has never experienced a bolt breakage. His weapons are presumably properly gassed though.

I knew it was relevant, from past readings on the subject, but still neglected to mention that it was indeed a lug next to the extractor. That aspect of the failure didn't surprise me, and it lends credibility to the idea that it wasn't a problem with the bolt.

I appreciate the breakdown of MPI and HP inspections, Rustpot. The minor machining errors or defects you mention: would they be visible to a person's eye? If they would be, I reckon it would take a QC element hand inspecting every piece, to find them. That's probably not something that's practical.