When ARs Fail — Busted Bolt Incident
Our friend Dennis Santiago was recently conducting training for a Southern California Police Department. During a training session one of the unit’s AR15s stopped functioning. The problem — the bolt in the AR rifle broke in half. Dennis states: “They ran the gun dry, broke for lunch, shot it again. They don’t like that. I personally like to flood the bolt wet with lube on training days. It prevents stuff like this. Given that, it’s a simple remove-and-replace fix.”
Here are some of the more interesting comments about this parts failure — an AR bolt that literally sheared in half:
“If I was a betting man [the steel] wasn’t made by Carpenter Steel. They are one of the few companies that use the correct [milspec-steel, C-158] called out on the drawing which they coincidentally developed. Most of the other companies that make [AR Bolts] use different steel with the same heat treat specification as what is called out which gives them the potential of being a little on the brittle side at the upper end of the tolerance. When it comes to the AR platform, bolts are probably the only part of the entire gun where I must admit to being a little bit of a brand snob.” David O’N.
[Editor’s Note: Actually Carpenter Steel does not make AR bolts. They are a steel supplier, and yes Carpenter did develop the original C-158 steel for AR bolts. Here is a contrary view, claiming that AISI 9310 Steel is actually stronger than milspec Carpenter C-158: 9310 Steel for AR Bolts.]
“Dry bolt and carrier shouldn’t cause that. Looks like a big pressure spike. What kind of ammo…?” Guy G.
Reply from Dennis: “55gr factory ammo.Piles of it.”
“Is that the new two-piece bolt everyone’s been talking about LOL?” — Darren R.
“Let me guess…the PD called you in cause they didn’t know why it stopped going ‘pew pew’?” — Jim O.
Reply from Dennis: “I was there today for qualifications. It broke during the rifle phase. Simple enough to fish the bolt parts out of the action. The training didn’t miss a beat. I have an armory full of the things.”
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Tags: AR Bolt Carrier, AR15, Bolt Failure, Broken Bolt, Dennis Santiago
A lot of really saavy AR experts will tell you that ARs don’t have to be run very wet at all like other experts say. They run lots of rounds through them too. Might be another cause.
My personal experience is a lot different.
I swipe the friction areas on my AR bolt with a little Super Lube grease and while I don’t do accurate round counts, I would guess it is in the hundreds of rounds and several months between cleaning and re-lubing. This is my fourth AR in 34 years and I have never broken a bolt.
Nickelboron
It could be a simple case of improper heat treatment. Make them too hard and they will fracture. Inspection is supposed to pick this up.
It’s a weak spot in the design. Even Colt bolts break. With lube, too much is better than not enough. However, once you have enough more isn’t better. I like grease on the bolt and cam as it seems to cook off slower. I think how much the smoking lube burns your eyes is a good criteria for which brand to use.
I tell what an old KS gun dealer would tell people about broken leaf springs in shotgun. Stop using WD40, it break those springs.
Can an AR run dry? Sure. Should you, no. The lube helps migrate out fouling and dirt and debris. A wet gun with dirty lube is better than a dry gun with fouling and dirt.
EDITOR: Actually excess lubrication, combined with poor cleaning practices, can be just as bad — maybe worse — than under-lubrication. We have worked with dozens of ARs. The #1 malfunction cause that we observed in semi-auto ARs was gunk in the ejector recess and the extractor pivot recess. This was a combination of lube, carbon and tiny brass particles. We suggest using the correct lube on high friction areas, cleaning the recesses “early and often” and do NOT run your gun “dripping wet”.
comon prblem for handme down rifles from military
I cringe when I see people hose down an AR with spray lube into the ejection port. Grease on the cam pin and a little bit of lube-de-jure on the bolt carrier rails and bolt tail, is all it needs. Hosing it down does a poor job of lubricating the things that need it, and puts lube in lots of places where it does nothing but hold onto gunk. Would you rather roll around in sand or dirt when you are soaking wet or dry?
Nature of the beast. Short gas tube = Over gassing. Chamber pressured up while bolt is trying to unlock. Adjustable Gas block?
If there is friction involved, lube it. The AR doesn’t need massive amounts, if it’s evenly coated, it’s good enough. The bolt lugs, contact with cam and bolt carrier can benefit from some decent high temperature grease, but again, no need to drown the damn thing. A little dab’ll do ya.
Lloyd nailed it. Over gassing was a likely factor.
The BCG and the barrel extension are the brains of the AR system. Not only do you want a bolt of the proper material spec, but it needs to be heat-treated, bead blasted and magnetic practical inspected for maximum strength/safety.
The carrier and the barrel extension should also be of the proper material.
Another critical part is the cam pin and it should not be overlooked.
What caused this failure? My guess is the material. We have no idea how many cycles the bolt has had and even with proper material, heat treat, etc…they can fail if beyond service life.
When I buy parts the entire BCG lock up parts is where I always verify quality.
By looking at the finish wear on the charging handle and broken bolt, it’s a simple matter of bolt head failure after thousands of rounds in a well-known weak/failure point on the M4 design. The rifle is a semi-auto so it’s probably on the upper end of the high round count before the failure happened. Not much to do with lubing at all.Metal fatigue and high pressure after thousands of cycles.
Has anyone done a simple test like doing a RC test on the bolt, case hardness and depth of case and core hordness. Fire 1000 rounds then test again and every 1000 rnds till 4000 have been fired. You will see a difference. The impingement design sends hot gasses into the bolt expansion chamber and this could start to reduce its core strength. Just a thought. Different ammo chemistry could also make a difference.