Help, broken 5.56 bolt and worn cam pin

[Casper77]

I tore my 5.56 AR down to clean it after a range session and noticed the cam pin had a groove worn in it. I've seen that before but this groove had a rough ridge on it which I hadn't seen before. Looking at the bolt I noticed that a small piece had actually broken off on the edge of the cam pin hole. The bolt is stamped MP, the AR has a 14.5 inch barrel and a carbine length gas system. The upper only has about 600 rounds through it, all have been factory Federal ammo. Does anybody know what could have caused this? By what you can see in the pictures is the bolt unserviceable? The carrier doesn't seem to be damaged in any way but could it be the culprit somehow?

[cory]

That's weird. Is that a nitride bolt?

[SG4247]

Need to can the bolt and cam pin.

I just saw one broken in half in the same area two weeks ago.

For $35 I will send you a new .223 bolt assy!

No need to risk it.

[Casper77]

That's weird. Is that a nitride bolt?

I'm not sure if it's nitride or not. The upper is an LMT but I don't know if the bolt came from them or not. Is there a way to tell if it has the nitride coating?

[Casper77]

Need to can the bolt and cam pin.

I just saw one broken in half in the same area two weeks ago.

For $35 I will send you a new .223 bolt assy!

No need to risk it.

I agree, I'll just replace the bolt and cam pin. Wish I knew what caused it though. SG4247 do you have an online store or just have one on hand? You can PM me the details...

[SG4247]

PM sent!

[LRRPF52]

That chip on the cam pin hole will probably act as a stress riser, and propagate the fracture until the bolt cracks all the way through the cam pin hole.

This was the most common failure point on the early AR15's.

[Casper77]

That chip on the cam pin hole will probably act as a stress riser, and propagate the fracture until the bolt cracks all the way through the cam pin hole.

This was the most common failure point on the early AR15's.

Roger, I will replace it for sure. Thanks for the help everybody.

[LRRPF52]

I've broken 5.56 bolts before. My first one was after 10,000 tortuous rounds through one of my 3-Gun carbines. It had a 16" CLGS barrel, so that gas system is really hard on bolts.

It cracked all the way through one side of the cam pin hole.

[Casper77]

It just baffled me that it did it so early on in it's life. The edges of the cam pin hole are all pretty sharp, no bevel at all, like I'm used to seeing on my service rifle.

[LRRPF52]

It just baffled me that it did it so early on in it's life. The edges of the cam pin hole are all pretty sharp, no bevel at all, like I'm used to seeing on my service rifle.

Sounds like one of these recent after-market bolts run through by vendors who are catering to businesses that don't know their metallurgy.

A shot peening could have exposed that issue. If you leave sharp edges like that at the cam pin hole, it will likely crack. That bolt might actually run fine and not crack anymore, or it might crack completely.

You really see what is garbage and what holds up in high volume carbine courses. Many of the big brand guns that sell the most volume will break bolts within a few mags on Day 1 morning, while others will last for tens of thousands of rounds.

With companies like BCM, Colt, DD, FN, LMT, ArmaLite, and KAC, the guns run and run like they like being shot that way.

Funnily enough, the PSA premium stuff is holding up well for full auto rental at Hendersen Defense out in Nevada. I used to buy surplus M16 bolts and carriers for builds because I knew they were vetted through Colt's TDP and Mil-spec process, and those have never given me a problem.

If anyone thinks these parts are all the same, I can tell you after 28 years of living with, breathing, eating, and sleeping AR15's, there are substantial differences in imitation parts, versus ones that meet the Mil-spec/Technical Data Package.

One thing about cam pins is that they are supposed to be made from a certain type of steel, heat treated to certain hardness, and are phosphated, then have a dry film lubricant baked onto them. You will see some peening and excessive wear on cam pins that are very loose-fitting in the bolt, especially on CLGS guns with ports that are too large.

[Casper77]

A shot peening could have exposed that issue...

With companies like BCM, Colt, DD, FN, LMT, ArmaLite, and KAC, the guns run and run like they like being shot that way.

PSA premium... surplus M16 bolts and carriers...
If anyone thinks these parts are all the same, I can tell you after 28 years of living with, breathing, eating, and sleeping AR15's, there are substantial differences in imitation parts, versus ones that meet the Mil-spec/Technical Data Package.

One thing about cam pins is that they are supposed to be made from a certain type of steel, heat treated to certain hardness, and are phosphated, then have a dry film lubricant baked onto them. You will see some peening and excessive wear on cam pins that are very loose-fitting in the bolt, especially on CLGS guns with ports that are too large.

Nothing like learning by doing right? Thank you for the wealth of knowledge. I'm going to research cam pins some more as I think that combined with the bolt weakness led to this failure. There is a definite groove with a rough ridge worn in the pin. When I replace the bolt I will keep a close eye on it and see how she wears in.

[Casper77]

A quick shout out to SG4247. Not only did he give me a great price on a new bolt assembly but he included a new cam pin. Thank you, Sir! Time to get back to shootin stuff.