AR-15 Bolt Sheds Lugs — Can You Figure Out What Happened?
Black Rifle Gone Bad…
Take a close look at this AR-15 bolt. Notice something missing — namely all the lugs? A healthy AR-15 bolt has seven (7) bearing lugs (plus an extractor hump). For all seven lugs to have sheared “clean off”, something serious must have happened to this bolt assembly. The folks at Brownells published this “lost luggage” image on Facebook to spur discussion. So, you AR experts out there — what do you think caused the problem here? Was it over-pressure, metal defect, headspace problem, gas system malfunction (or some combination of issues)? Post your theories in the comment section below…
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Tags: AR-15, Black Rifle, Bolt, Brownells, Lugs
The result of a metric bolt used on
an SAE rifle…
some one have used BCG like tool to take out barrel extension………….
What really happened” Please give us the solution.
The contact marks on the end of the carrier show it did not fail on the first shot.
All lugs fractured in a single brittle mode event from a single overload. Not fatigue or progressive.
What do the lugs on the barrel nut look like?
Bolt looks consistently hard and brittle all the way thought, what are the specs for hardness gradient?
Can a 300 AAC/Blackout round be chambered in a 5.56 chamber and the bolt closed?
Interesting!!
Mike
The lack of “burn marks” on the face of the bolt would indicate that the pressure involved did NOT result in the case, and thus the primer pocket, “bulging”. If the case head had bulged,or the primer “failed”, there would have been a nasty little ring around the striker hole and/or, the edges of that hole would have been gas-eroded.
If that is a hard-chromed bolt (and carrier), perhaps it skipped past the “oven” stage after chroming. Result, the hydrogen that bonds with the steel during hard-chroming, causes hydrogen embrittlement resulting in fractures as shown. The crystal structure at the fractures could also indicate a possible problem with “over-cooking” in the heat-treatment / hardening stage.
Maybe its a hoax.
Guy: This image was originally published by Brownells to spur discussion. We are still waiting for Brownells to provide a full analysis.
temping on the bolt or bad head space
It would have split the carrier if it was pressure. Looks like a bad bolt to me.
it looks dead on for an electroless nickel boron coated bolt carrier group, no zinc, no electroplating. Due to the catastrophic failure of the bolt as a whole, a good assumption would be excessive case hardening. The process of NiB plating dose not involve temperatures that would cause such an embrittlement. Overpressure would have taken out the extractor as as well as sheered the lugs off the bolt.
Reloaded cartridge, probably fast pistol powder in 223/556 casing is my best guess.
Hydrogen embrittlement would be my second guess. They should have baked the part after plating to deal with this issue.
But whatever the reason, please do tell us what happened here.
That looks like the bolt from my rifle. 24.0 grains of Varget, CCI BR4 primer, new winchester case. The projectile was a 77 SMK seated to mag length.
I was breaking in a barrel. I had 80 rounds from that lot through the barrel, then suddenly the bolt wouldn’t lock back. The fired case was normal. In fact all the cases were normal.
Charles
The lugs were engaged in the chamber at the time of the malfunction. However, extreme chamber over-pressure possibly due to improper loads caused the force on the bolt to exceed the strength of the lugs. This happened before the bullet passed the gas port in the barrel. Otherwise, the DGI system would’ve unlocked the bolt before destroying the lugs.
New or old it is definitely looking metal fatigue. Looking at the break, the metal grain appears not normal and looks like it had a porosity issue from the get go before it was machined. Hydrogen embrittlement on top of bad metal and an over pressure may have all compiled to cause an issue like this.
300 bo 110gr or 125gr loaded into a 5.56×45 chamber the bolt appears to be brand new with very light carbon build up in the machining marks, the ejector shows no signs of finish were. there is heavy brass build up on the bolt face, and the gas key base shows evidence of high speed flowing brass. PS yes 300 will fit in 5.56 chamber I checked it out using SAAMI specs.
faulty bolt, hydrogen embrittled, poor heat treatment(overheat), improper alloy, etc
We will never known for sure
The crystalline structure of the metal worries me.
Sharps Reliabolt made from S7. It’s the wrong material for the application and results in brittle failure, especially at sub zero temps. This is may have been the first round on a 20-30 degree day.
the crystal structure of the bolt suggests that the lugs all failed catastrophically probably due to being far too brittle. The extractor lug has deformed in a more plastic manner which supports this hypothesis. Id did not fracture.
That bolt was probably used in a parted out built weapon. The builder used various components purchased and manufactured from various places, likely. In doing so, he/she simply overlooked the lug spacing requirements, e.g. metric vs SAE. Those lugs likely failed due to not being matched to the barrel lug spacing. If that is correct, the lugs will indeed rotate, but will in turn be about 40% engaged.
5.56 in a .223 chambered rifle
Over-pressure
I think it had a round or two that was reloaded too many times and the primer pocket got weak over time. When fired, the shell casing went back and sheared of the lugs.
I don’t know why some commentors are saying an SAE and Metric “mix-up” happened… The bolt is an AR-15 – There is no Metric version. Only a single standard!
I really hope those guys don’t work on guns. (◔_◔)
Improper Headspace
Failed cam pin resulting in bolt not disengaging from barrel extension
First shot squib load. Second shot this…
I wish there was a way to upload the photo of the brass which was the result of a catastrophic failer brought on by firing my bolt with all but 1 lug sheared off. That 1 lug is definitely gone now. Upper sustained notable damage, dust cover destroyed as was the surefire 60rnd stick mag being used at the time. But this brass…… ive never seen anything remote close to this kind of damage. The back pressure literally imploded the empty brass on itself. Kinda scary. Very scary actually.
Looks like a pressure test on the lugs of a bolt.
How much can the lugs take before they give way.
Looks to clean to be a field failure.