Barrel extension failure

[Slater]

From the TFB site. Anyone ever encountered this before?:

https://www.thefirearmblog.com/blog/...ation-failure/

[Ned Christiansen]

Hain't seen that one before. Not a shocking failure mode really-- upon seeing it I wondered why it is not more common.

That BE is pretty non-standard in that the bolt pass-through cuts are full-radiused at the bottom, as if they were done with an end mill as opposed to having been broached in. I don't see where that is by itself a bad thing, in fact in theory it could be a good thing.

I want to say I see a popped primer but it looks too small. But it would not be surprising to see a popped primer in conjunction with another failure

[Clint]

It's not common because a mil spec BE is stronger than the bolt.

Looks like bad heat treat on the extension and possibly small/no radius on the transition to the lugs, creating a stress riser.

[vicious_cb]

The barrel of this rifle has a marking “BHW” which stands for Black Hole Weaponry. Apparently, BHW is now the subsidiary of Columbia River Arms.

I think I found the problem. Buy as good as brands, get weird problems. I guess thats the reason why every quality barrel Ive bought has a hardness test dimple mark on the BE, checking things for defects is a thing, who knew?

[Todd.K]

With the price and availability of good known quality barrel extensions, the non standard machining is a bit of a red flag.

[Ned Christiansen]

I've never seen a Rockwell tester dimple on a BE than I can recall.... or they all have it and I've stopped noticing :-)

Well normally the broached cuts for bolt lug pass through go to a diameter that is smaller than the ID of the BE. These cuts, of necessity, because of the full radius of the cuts, have to go further. That makes it theoretically weaker and the full radius makes it theoretically stronger. Not saying it's a wash, I really don't know. Like Clint said, maybe not enough radius in the corner or the fact that it was interrupted by the radius cuts.

[Milspec78]

I would like to see the spec's on that part....

[curious1]

That style barrel extension has been around a while now. I have only seen it on one companys barrels. I ran into it more then a few years ago when I had a bolt that would not lock in. The outside bolt lug corners were hitting the bottom of the U portion of the extension.
I pulled the barrel I have with the similar style rounded lug cuts out of the junk can. The barrel it is on is marked 5.56 and has a cast front sight base. The extension is marked with something that looks like a circle with a Y inside it stamp on the face of it.

[1168]

I've never seen a Rockwell tester dimple on a BE than I can recall.... or they all have it and I've stopped noticing :-)

It is certainly not universal. The only barrel I own that has it is a DD Mk 18 barrel. And I have no reason to claim the dot on the extension is for hardness testing, but it does have a dimple.