Bolt Failure today..

[26 Inf]

A bit of a bump, but just to serve as reference: this guy also had a nib bolt break at an extremely low round count. Coincidence perhaps. Are carriers coated with hard chrome just as susceptible for improper heat treatments?


Info
http://www.reddit.com/r/ar15/comment..._at_the_range/

Not to sure on the round count he provided - 300 to 400. Manufacturer reportedly Black Rain. Look at the lugs on original image size, I don't think that is a 400 round bolt.

[LRRPF52]

I was going to ask about receiver face squareness as well. Did you ever get the materials analysis back? I don't see it in the thread.

I think the biggest issue is that we are using a bolt geometry that was never meant for the cartridge and pressures we are shooting through the AR15 ( Since the AR15 was engineered around the .222 Remington, which is a 50,000psi SAAMI MAP cartridge). If you look at the dimensions of critical parts, pressure vessel ID and OD of the tennon, the Factor of Safety comes out to a solid 2 with .222 Rem. @ 50ksi.

When the Army Ordnance Board moved the goal posts for the steel helmet perforation requirement from 400yds to 500yds, the .222 Rem. "only" penetrated one side of the helmet. Stoner asked Remington if they could blow the shoulder forward, use a FMJBT of 55gr weight, and go to a faster powder to push the velocity. Hence was born the .222 Remington Special, later type classified as the 5.56x45. Pressures went from a max of 50ksi, to anywhere between 55-58ksi.

With M855, pressures can go to 62ksi. That's 12ksi more than the original design parameters of the rifle. Now go to a CLGS, suppressed, use 8620 with bad metallurgy, and lugs will shear, or bolt bodies will crack at the cam pin hole. It takes very good metallurgy to account for this problem with the AR15/5.56 chambering, not only in the bolt, but the barrel and barrel extension.

In discussing this challenge with ME's who specialize in firearms, the concern has been raised as to the effects of the NiB surface impacting a phosphated 8620 case hardened steel barrel extension, since the spherical nodes of boron will win against the rough surface grain structure of the steel.

[MegademiC]

So, I feel like a prick.

No results yet. Our metallurgist keeps telling me he'll do it "this week", for the past 3 months. Same with the Chemlab manager.

We actually ran the boron once, but there was too much interference because he didn't run it on the correct setting. Based on what it took to dissolve it, I'd guess NiB

Our metallurgist has a piece mounted for a micro and hardness testing, but just hasn't done it yet.

All I can definitely tell you so far is that the surface was stained black - Caustic cleaner, ultrasonic hexane bath, and a dip in ~15% HCL and it would not shine up.
Also, the coating was about 350μin thick.

My theory is that it is not the coating itself, but the process. Perhaps the coating was not done properly and causes hydrogen embrittlement of the bolt?

[Junkie]

Not to sure on the round count he provided - 300 to 400. Manufacturer reportedly Black Rain. Look at the lugs on original image size, I don't think that is a 400 round bolt.

Or it could be that improper heat treat lead to soft lugs

[Uni-Vibe]

Yep - my feeling is that 5,000 is the top side for bolt life, and less for a suppressed or machine gun. I'm pretty sure it's even written somewhere.

I've replaced bolts at around 5k in the past. I've since stopped maintaining a replacement schedule or even really keeping round counts, but this is a reminder that it should probably be maintained with a designated hostility gun, at least. Training/fun guns can be ridden to failure without much consequence, obviously.

Yeow! Back in the Seventies I used to shoot with some ROTC guys. We shot Vietnam era M16A1. We cleaned those guns "white glove" clean; spent three hours doing it after shooting. We never lubed them. (It was ROTC, remember?) And they had God knows how many rounds through them from wartime use. 40,000? 150,000? Who knew? Never a problem even in the absence of proper lubrication.

[wegner426]

I did a little research, and the link below seems to indicate that Melonite won't cause hydrogen embrittlement. I just purchased a Seekins Precision BCG that is Melonited, so will see how it works when it comes in. Not sure what the board consensus is on Melonited BCG's, i didn't find all that much in a google search on the Seekins BCG.

http://www.met-sol.com/assets/Indust...C_Brochure.pdf

[WS6]

I was going to ask about receiver face squareness as well. Did you ever get the materials analysis back? I don't see it in the thread.

I think the biggest issue is that we are using a bolt geometry that was never meant for the cartridge and pressures we are shooting through the AR15 ( Since the AR15 was engineered around the .222 Remington, which is a 50,000psi SAAMI MAP cartridge). If you look at the dimensions of critical parts, pressure vessel ID and OD of the tennon, the Factor of Safety comes out to a solid 2 with .222 Rem. @ 50ksi.

When the Army Ordnance Board moved the goal posts for the steel helmet perforation requirement from 400yds to 500yds, the .222 Rem. "only" penetrated one side of the helmet. Stoner asked Remington if they could blow the shoulder forward, use a FMJBT of 55gr weight, and go to a faster powder to push the velocity. Hence was born the .222 Remington Special, later type classified as the 5.56x45. Pressures went from a max of 50ksi, to anywhere between 55-58ksi.

With M855, pressures can go to 62ksi. That's 12ksi more than the original design parameters of the rifle. Now go to a CLGS, suppressed, use 8620 with bad metallurgy, and lugs will shear, or bolt bodies will crack at the cam pin hole. It takes very good metallurgy to account for this problem with the AR15/5.56 chambering, not only in the bolt, but the barrel and barrel extension.

In discussing this challenge with ME's who specialize in firearms, the concern has been raised as to the effects of the NiB surface impacting a phosphated 8620 case hardened steel barrel extension, since the spherical nodes of boron will win against the rough surface grain structure of the steel.

Exactly. This is why I am in the "Mil-spec only" camp when it comes to a BCG. Right? Wrong? Regardless, it's KNOWN quantity, and it's not shabby!

[HD1911]

Exactly. This is why I am in the "Mil-spec only" camp when it comes to a BCG. Right? Wrong? Regardless, it's KNOWN quantity, and it's not shabby!

Agreed.

[Cumminspowa]

I did a little research, and the link below seems to indicate that Melonite won't cause hydrogen embrittlement. I just purchased a Seekins Precision BCG that is Melonited, so will see how it works when it comes in. Not sure what the board consensus is on Melonited BCG's, i didn't find all that much in a google search on the Seekins BCG.

http://www.met-sol.com/assets/Indust...C_Brochure.pdf

I talked to Seekins a while back asking them some questions, which they were very helpful with. But they use standard hpt/mpi C158 mil spec phosphate coated bolts and then the carrier and gas key are what they do Melonite treatment on. I'd say that is a very good/smart combination imo.

I think mil spec is a definite quality that can't be argued with, but I do think however there are innovative coatings/materials that can be used to produce a superior part.

[GeorgeB]

The only hardware failure I've had on an AR was an extractor claw breaking off.

Keeping fingers crossed.