Question and intro - Do barrel extensions ever fail?

[meganeura]

First, an introduction...I am pretty new to the AR game, but long enough to have learned a lesson or two. I bought a Bushy as my first AR and soon discovered it had a .223 chamber instead of .556. Pressures were so high I could feel the gas escaping with my left hand from under the FSB. Robb Jensen and his Christiansen reamer took care of that. But I eventually sold that and ended up buying an LMT. I also have a Colt 6920 and a BCM M4 MOD2; so I have learned a lot by just lurking here for a while. Buy top tier stuff and you won't regret it, although I admit it is a sometimes a source of amusement for me to watch someone try and justify their choice of inferior products. I have always believed the gun-ammo combination is a system and as such I use only high quality ammo (IMI M855 almost exclusively) whenever possible. All the guns have Aimpoint M4S sights, which to me are outstanding sights.

I am not a police officer, service member, operator, or anything like that...and will never pretend to be. I work for the US Army as a materials scientist (civilian) and between that and kids I don't have much time for much else. I shoot when I can, just for fun. One day I hope to take a carbine course though. Until then, my plan when visiting here is to keep my ears open and my mouth shut for the most part. By the way my handle/avatar is named after the giant Carboniferous-era dragonfly with the 3 foot wingspan, in case anyone wondered.

Now for my question - I know from an email query to BCM that at least their barrel extensions are type 8620 steel. The barrel extension has plenty of stress placed on it during firing. The lugs on the barrel extension seem to be vulnerable just by virtue of their geometry. How is it that those never break, whereas bolts breaking seems to be a regular occurrence?

[Robb Jensen]

You generally only ever see damage to barrel extensions when the rifle kBs (out of battery detonation).

[devinsdad]

Most of the bolts that break do so at the locking lugs. Specifically the lugs on either side of the extractor. This is due to the uneven amount of stress applied when firing. The extractor lug is smaller than the others and as such does not provide the same amount of locking force as the others do. Those near the extractor have to do their job and part of the job of the extractors lug. In an effort to combat this from happening, Armalite relieves the lug opposite the extractor to lessen the chance. It would be interesting to see a study of bolts to assertain if this actually does what it's advertised to. Manufacturers using improper steels, heat treatment and lack of testing are the main reasons of bolt failure. Some are traced back to improperly loaded ammunition, but you will usually see more damage than just the bolt.

[meganeura]

Thanks for the reply Robb...

For those who have not been there, I can say I highly recommend VA Arms. Very knowledgeable staff and an excellent selection of guns and gear. It's almost a spiritual experience every time I go in there. If I didn't have kids in college :fie::suicide: I'd spend a lot more time and $$$$ in there...

I also appreciate the enormous amount of expertise here, and I salute those folks here who are deployed for the sacrifices they are making. I also appreciate the no nonsense way this place is run. Very refreshing.

[Eric D.]

I didn't think this could happen on an AR. All the kb's that have been talked about here have been traced to an ammo problem or barrel obstruction.


You generally only ever see damage to barrel extensions when the rifle kBs (out of battery detonation).

[meganeura]

That's why asked. I have never heard of a barrel extension failure either, and it has lugs that concentrate stress to some extent. Not a terribly profound question - just curious. Obviously the bolt has more concentrated stresses due to its design and function, and hence a greater failure rate.

[Robb Jensen]

I didn't think this could happen on an AR. All the kb's that have been talked about here have been traced to an ammo problem or barrel obstruction.

It can happen but it is very rare. You are correct in that most kBs are from squibs or using the wrong powder when reloading.

[An Undocumented Worker]

Here ya go. Someone had an Oberland Arms AR that broke a barrel extension, though it seems that poor manufacturing was to blame for that.

http://m4carbine.net/showthread.php?t=50279

[meganeura]

After looking at the Oberland Arms thread and their website I think I'll stay with Colt/BCM/LMT :smile: ... Not everything German made is the best...

[fuse]

You generally only ever see damage to barrel extensions when the rifle kBs (out of battery detonation).

I thought it was basically impossible for the AR-15 platform to fire out of battery.

Edit: I see this was discussed above. Please ignore.

[Robb Jensen]

I thought it was basically impossible for the AR-15 platform to fire out of battery.

Edit: I see this was discussed above. Please ignore.

The last time I saw one it was theorized that a piece of the bolt, bolt catch or mag feed lip found its way between the bolt face and case head and detonated the round. The bolt was destroyed as well as the upper and lower receiver.

[Failure2Stop]

The last time I saw one it was theorized that a piece of the bike catch or mag feed lip found its way between the bolt face and case head and detonated the round.

Bolt catch?

[Robb Jensen]

Bolt catch?

Yes there was a small piece of the bolt catch and mag feed lip missing and the bolt has shed two of it's lugs.

[meganeura]

My apologies by the way. I see I should have put my intro in the sticky up top. In all my lurking I didn't see that.

[JSantoro]

No worries, they're a like-to-have, not a have-to-have.

[meganeura]

One other thing I have been wondering, and not able to find in searching, is what type of steel the cam pin is made out of. I have a hunch its 8620. I'm pretty sure most bolt carriers, good ones at least, are 8620.

[Clint]

Now for my question - I know from an email query to BCM that at least their barrel extensions are type 8620 steel. The barrel extension has plenty of stress placed on it during firing. The lugs on the barrel extension seem to be vulnerable just by virtue of their geometry. How is it that those never break, whereas bolts breaking seems to be a regular occurrence?

You answered your own question.

The bolt lugs and extension lugs are placed under the same stress during firing.

But the extension lugs are quite a bit stronger due to their size and geometry.

The bolt lugs, being the weaker part, break first, limiting the force on the barrel extension.

This is the same situation as a chain made out of 3/8" and 1/2" links.

When pulled to the breaking point, the smaller links will always break first and the larger ones will never break.

The Oberland extensions were apparently messed up in heat treat and therefore, much weaker than they were supposed to be.

[constructor]

You answered your own question.

The bolt lugs and extension lugs are placed under the same stress during firing.

But the extension lugs are quite a bit stronger due to their size and geometry.

The bolt lugs, being the weaker part, break first, limiting the force on the barrel extension.

This is the same situation as a chain made out of 3/8" and 1/2" links.

When pulled to the breaking point, the smaller links will always break first and the larger ones will never break.

The Oberland extensions were apparently messed up in heat treat and therefore, much weaker than they were supposed to be.


Agree, extension lugs are much larger and the extension outer rim is much thicker than the web of a bolt. The lugs of the bolt themselves don't break, the web they are attached to breaks.
All of the bolts I have seen fail do so at the cam pin hole except for the Grendel/ 7.62x39 bolts.

[constructor]

Most of the bolts that break do so at the locking lugs. Specifically the lugs on either side of the extractor. This is due to the uneven amount of stress applied when firing. The extractor lug is smaller than the others and as such does not provide the same amount of locking force as the others do. Those near the extractor have to do their job and part of the job of the extractors lug. In an effort to combat this from happening, Armalite relieves the lug opposite the extractor to lessen the chance. It would be interesting to see a study of bolts to assertain if this actually does what it's advertised to. Manufacturers using improper steels, heat treatment and lack of testing are the main reasons of bolt failure. Some are traced back to improperly loaded ammunition, but you will usually see more damage than just the bolt.


Extractor lug? the extractor doesn't have a lug, that is a stiffener to support the hook of the extractor so it doesn't rip off due to the thin area at the groove of the extractor. That little rib does not touch anything when in battery.

[jmart]

Extractor lug? the extractor doesn't have a lug, that is a stiffener to support the hook of the extractor so it doesn't rip off due to the thin area at the groove of the extractor. That little rib does not touch anything when in battery.



Armalite deprecates the lug 180 degrees opposite this extractor, so in an Armalite bolt there are 6 evenly loaded lugs, unlike a regular bolt that has 7 loaded lugs, but the lugs adjacent to the extractor are disproportionatly loaded. In Armalites design, each lug's loading goes up slightly, but by evening out the load, the load on each lug adjacent to the extractor, which are lugs most often seen to break in conventional bolts, is reduced.

[constructor]

Armalite deprecates the lug 180 degrees opposite this extractor, so in an Armalite bolt there are 6 evenly loaded lugs, unlike a regular bolt that has 7 loaded lugs, but the lugs adjacent to the extractor are disproportionatly loaded. In Armalites design, each lug's loading goes up slightly, but by evening out the load, the load on each lug adjacent to the extractor, which are lugs most often seen to break in conventional bolts, is reduced.
I know that but I didn't say anything about the lugs except that the knot on the back of the extractor is NOT a lug it is a stiffener.