Gunsmithing Unsupported case wall = big Kaboom?

[Ksracer]

Long story, I'll try to keep it short. How much of the brass can stick out of the chamber before the case wall/web that's left unsupported can't handle the pressure and it ruptures?
Is there a hard number not to exceed?
The rifle in question is a Defiance chambered in 6BRa using Lapua brass. The problem arose when the original barrel was replaced with one that had .035 deep chamfer/feed cone cut at the chamber mouth, in addition to the .150 deep bolt nose recess. There was a case failure after the first ~ 30 rounds that blew the bottom of the mag apart. The rupture lined up perfectly with the deepest part of the chamfer. If you look closely at the pic, you can see the brass and where it failed. We sectioned several pcs of the brass looking for signs of case head separation and didn't find anything else that would have lead to this failure.

 

[Steel head]

That seems like an excessive chamfer Depending on the angles it’s cut on.

the only thing I ever see protruding and un supported it the extractor grove.

 

[cwinner]

Who in the world would cut something like that... Lucky it took 30 rounds..... should have let go on the first round fired!

Would be interested what the brass looked like and if it caused any damage to the bolt/action.

 

[Mordamer]

I guess we can say for certain that .150 recess and .035 deep chamfer doesn't work well for a Lapua BR case. Haha

 

[Mordamer]

To add more information to this thread, what was the load the shooter used?
I guess there is a chance of a bad load?
Do the other pieces fired show visible expansion in the unsupported section of the case web? If they don't I think it might point to one bad load causing the problem.

The chamfer looks excessively deep, but .035" isn't much if it makes the difference between a safe chamber job and one that blows up. A more reasonable chamfer would be .010 deep at a 30 degree angle from perpendicular. Maybe the extra .025" of chamfer depth is the cause and maybe not. If it was a hot load that caused the problem the case is going to rupture at that exact same location regardless.

 

[Ksracer]

Some more pics of a cutaway, and some fired brass. There's definitely a step where the brass was exposed. It's not as easy to see with your eye, the camera really accents it.
A little more background. I cut the original barrel back in March for the owner. When the PreFit came in, he briefly swapped it on for some break in, and then reinstalled the original until it burnt out. He didn't notice any problems, but wasn't really looking at the base of the brass for steps like that either. I suspect the case that ruptured was probably the first piece that had been fired in that barrel for the second time. He has 400pcs of brass, and there's no way to keep track of it, (PRS matches), so that's just suspicion on my part.
He's running 30.3gn of 4895 behind a 105 class bullet, so it's a very normal load. Velocity's fell in line with everything else he'd shot, and the primers looked normal with no signs of ejector marks.
I'm hoping some of the guys who have been formally trained as smiths will have an actual spec for this. The company who produced the barrel denied all responsibility and had a laundry list of of excuses, ranging from overpressure, bad fireforming, poor reloading, and headspace issues.

Edit. Substituted cutaway pic for one with virgin BR brass, firmly seated.

 

[Supersubes]

Yikes! From your cutaway, that’s way too much imo. I like a generous chamfer, but never exceeding the solid portion of the case. The solid section of the brass below the case volume varies a bit depending on the cartridge and brass make.

What’s the depth of the counterbore?(disregard, missed it in the original post). Looks a bit deep to me, and if so, would exacerbate what could be an acceptable chamfer.

Edit: .035 all by itself doesn’t stick out to me. There’s too many other dimensions that need to be taken into account that are receiver specific. For example one receiver has a relatively shallow bolt face, and prior to the chamfer, the tail end of the barrel is hanging way out over the extractor groove of the case. In another receiver that’s maybe a little deeper, and maybe it’s been trued excessively, the case ends up protruding a bunch more, which wouldn’t allow for much chamfer.

 

[Ksracer]

After looking at that cutaway pic closer, I'm betting that brass isn't fully seated in the chamber. ( The customer took it) I sectioned some virgin BR and it looks better, but the failure and the marks on the other brass still all line up with the bottom of the chamfer. BR has a .170 thick head. That chamfer probably would have been fine with a Creed @.200+. thick head.

Yikes! From your cutaway, that’s way too much imo. I like a generous chamfer, but never exceeding the solid portion of the case. The solid section of the brass below the case volume varies a bit depending on the cartridge and brass make.

What’s the depth of the counterbore?(disregard, missed it in the original post). Looks a bit deep to me, and if so, would exacerbate what could be an acceptable chamfer.

Edit: .035 all by itself doesn’t stick out to me. There’s too many other dimensions that need to be taken into account that are receiver specific. For example one receiver has a relatively shallow bolt face, and prior to the chamfer, the tail end of the barrel is hanging way out over the extractor groove of the case. In another receiver that’s maybe a little deeper, and maybe it’s been trued excessively, the case ends up protruding a bunch more, which wouldn’t allow for much chamfer.

 

[Easy_E]

The customer is doing the barrels swaps ? Does he have the go gauge to set the head space?

 

[Ksracer]

The customer is doing the barrels swaps ? Does he have the go gauge to set the head space?

Yes. He has a BR gauge that's ground .004 short.
I cut a shoulder on my barrel, and headspaced .003 short on a standard BR gauge.
The Pre-fit has a nut on it, so a gross headspace error was possible, but fired brass from both barrels measures identically to the datum line using a comparator tool. I believe he installed it correctly.

 

[Rubicon Precision]

Most cases, this would not be a problem. The BR cases have a much smaller web thickness than most and that needs to be taken into consideration when chambering them, especially when you already have a minimum of .150” unsupported in the counterbore.

Edit: in theory, receiver dimensions in hand while creating the breech face/counterbore/chamfer, .150” counter bore + .035” chamfer +.005 gap should be fine, but right on the edge. Random barrel nut counterbore barrel, all bets are off.

 

[Supersubes]

Just took this out of the lathe. This is exactly 35 thou deep at 45 deg. 6.5x47 though. Still short of the case floor.

There’s a little red line in there that projects the case floor out to the case edge. If anyone’s wondering about the goofy tenon , it’s for a Q Fix.

 

[Steel head]

Just took this out of the lathe. This is exactly 35 thou deep at 45 deg. 6.5x47 though. Still short of the case floor.

There’s a little red line in there that projects the case floor out to the case edge. If anyone’s wondering about the goofy tenon , it’s for a Q Fix.
View attachment 7168760View attachment 7168763

that looks fairly similar to the latest barrel I got.
All of the case wall is supported.

 

[Ksracer]

I had this one in the lathe on Sunday. Another BRa, except for an Impact this time, although the bolt/tenon interface is nearly identical. The bolt nose is .150 deep + .008 clearance to bottom of counter bore + .010 x 20* chamfer = .168 total depth, which puts it almost exactly flush with the case floor. This is the way I've always cut them, and have never had issues.

 

[LRI]

Long story, I'll try to keep it short. How much of the brass can stick out of the chamber before the case wall/web that's left unsupported can't handle the pressure and it ruptures?
Is there a hard number not to exceed?
The rifle in question is a Defiance chambered in 6BRa using Lapua brass. The problem arose when the original barrel was replaced with one that had .035 deep chamfer/feed cone cut at the chamber mouth, in addition to the .150 deep bolt nose recess. There was a case failure after the first ~ 30 rounds that blew the bottom of the mag apart. The rupture lined up perfectly with the deepest part of the chamfer. If you look closely at the pic, you can see the brass and where it failed. We sectioned several pcs of the brass looking for signs of case head separation and didn't find anything else that would have lead to this failure.
View attachment 7167273

Generally speaking:

From the face of the barrel tennon to the breech face of the chamber, the dimension should not exceed .155-.160". The case more or less is flush with that.

Assuming your action isn't goofy or "gun smitted" that's where you should be for a ball park number and you can expect it to work. If in real doubt, saw a case in half and slide it up the chamber. It the head is not supported, you have a bomb waiting to happen.

 

[cwinner]

I don't see the point in cutting any chamfer on any tennon with a bolt nose recess, it seems that the .150" recess would serve the propose of allowing the case to chamber easily enough...... i personally round the edge with a file slightly so it dosent scratch cases.

Chad said it best above, keep the case supported as much as possible. I'm guessing the original issue was caused by tolerance stacking..... all those .001"'s ad up!

I can see the need for a chamfer for flat faced bolts but not in one with a counterbore.