Oversize flash hole/case rupture

[Sixgun]

Just out of curiosity and I didn't see it mentioned...what was load and how far into the lands or off them were they seated?

 

[Dave Marshall]

Just out of curiosity and I didn't see it mentioned...what was load and how far into the lands or off them were they seated?

38gr of H4350 behind a 107 SMK. I didn’t measure, but the throat was toast. It has to be at least .040 off.

 

[fyrewall]

Just waiting to see what the borescope shows about the carbon fouling. Thanks for the blow by blow. I think Brownells makes some type of device for extracting separated headless brass. I have had some success with oversize bronze bore brushes. I don't know why a carbide drill bit was used to extract the soft brass stuck case. Sorry about missing the part where you measured neck dimensions. Damaging the chamber with that carbide drill would void any further chamber analysis. I take it that the headspace was excessive due to turning the carbide drill into stuck case and chamber - that must have been close to a 5/8 inch diameter drill.

 

[Dave Marshall]

Just waiting to see what the borescope shows about the carbon fouling. Thanks for the blow by blow. I think Brownells makes some type of device for extracting separated headless brass. I have had some success with oversize bronze bore brushes. I don't know why a carbide drill bit was used to extract the soft brass stuck case. Sorry about missing the part where you measured neck dimensions. Damaging the chamber with that carbide drill would void any further chamber analysis. I take it that the headspace was excessive due to turning the carbide drill into stuck case and chamber - that must have been close to a 5/8 inch diameter drill.

The drill bit was a last resort. I tried several other methods first. It was done by hand, so there is still plenty to see in the chamber.

 

[fyrewall]

The brass manufacturer has inspected the cases and my gunsmith has the barrel but hasn’t looked at it yet. Here is what I’ve got so far:

There is nothing in the characteristics of the brass that would be considered defective. The hardness is correct and the wall thicknesses are also within parameters. There is one ruptured case and one intact case. I am told the intact case is barely intact and that it was likely also over pressure. The larger flash hole is most likely due to the brass being deformed when it ruptured. The case head is actually bulged inward from gas venting around the primer and against the bolt face. This forced the primer pocket forward. The case body measurements are all normal for cases fired in a properly sized chamber. The case necks, however, show some possible issues. The case mouths measure .268 for the intact case and .272 for the ruptured case. The neck diameter also increases to .274 as it is measured toward the shoulder. Based on these measurements, it seems the case mouth was somehow constricted causing a drastic pressure increase in the cases.

Now for the barrel:
The barrel was a previously used 6xc cut with an older (smaller) reamer. The neck of the chamber was reamed with a .276 neck reamer for more reliable feeding. The barrel was then used to fire at least 2000 trouble free rounds. I needed a test barrel, so I had the chamber re-cut with a larger 6xcII reamer. This pushed the shoulder about .002-.003 forward. Headspace difference with the new chamber and new brass is .007-.010. It’s more than most would like, but not outside of SAMMI allowances.

The chamber has not been inspected with a borescope yet, but here is what I think may have caused the issue:
The brass previously fired in the old chamber was trimmed each firing to ensure a uniform length and chamfer. The trim length was set once and not changed. There could have been a small amount of carbon buildup in the freebore just forward of the case mouth, but not enough to cause problems. When the shoulder was cut slightly deeper, it allowed the case mouth of the new, untrimmed and longer brass to be pushed into the tight area of carbon buildup. This would cause a huge spike in pressure when fired.

My final shot (pun?) The head space for new brass is .007 - .010, and that is from .001 to .004 more than the .006 deemed max by many. One case was intact and was "likely over pressure". Was a separation incipient for that case; was the head expanded; was the primer pocket enlarged? Was the diameter of both cases measured; if the flash hole was enlarged by firing it would be likely the case diameter at that area would also be enlarged.

Attacking that soft stuck brass case with a carbide drill sounds sort of overkill. Unless the chamber shoulder was actually cut away with that super hard carbide drill a scratched up internal chamber shoulder would provide enough surface to make a headspace determination; this would have shown something. Case removal using the carbide drill was done by hand.

My guess is that the new brass had excessive headspace and was overpressure. It would be of interest to determine the extent of any carbon buildup to cause compression of a case mouth but excessive length of new untrimmed longer brass could.

As I get more exposure to this event, the more I think it was caused by a combination of a hot load & excessive headspace & over length brass & possibly, lots of bullet jam and neck tension.

 

[mrjdasr]

OP stated that the throat was pretty well shot out, so a carbon ring could have formed in front of the chamber where the new reamer wouldn't reach it
hard to believe a carbon ring could build up thick enough to crimp bullets in so tite that it created that much
over pressure
I hope a for sure answer is found. very fortunate nobody was hurt worse than they were
really looking forward for an update when it is available

 

[Rtheurer]

Tubb 2000 Rifle?

 

[Dave Marshall]

Tubb 2000 Rifle?

Yup, strong receiver and good design saved my face.

Should be good to go with a new mag well, extractor, and bolt catch.

 

[Rtheurer]

Sako Extractor and Coned bolt. Nope. not for me, I had the exact same thing happen to me in a 6BR in a Tubb 2000. Got rid of that death trap quick.

 

[243winxb]

"3 Rings of Steel" http://benchrest.com/showthread.php?54852-quot-Three-Rings-of-Steel-quot-Question-Discussion



Post 33 at link.

When you add a Sako Extractor to a Remington 700 style bolt, you TOTALLY negate the safety enginering placed into the design by Remington. Jackie Schmidt.

 

[Dave Marshall]

Sako Extractor and Coned bolt. Nope. not for me, I had the exact same thing happen to me in a 6BR in a Tubb 2000. Got rid of that death trap quick.

The coned bolt of the Tubb is a known problem only with the 6BR cartridge. It’s NOT a problem with the rifle design. The 6BR has a thinner web and the case head must be 100% supported in any rifle design. If it’s machined properly it’s not a problem at all. I know several shooters that have fired thousands of trouble free rounds of 6BR in their Tubb rifles.

 

[Rtheurer]

The coned bolt of the Tubb is a known problem only with the 6BR cartridge. It’s NOT a problem with the rifle design. The 6BR has a thinner web and the case head must be 100% supported in any rifle design. If it’s machined properly it’s not a problem at all. I know several shooters that have fired thousands of trouble free rounds of 6BR in their Tubb rifles.

no disrespect, but they can have at it. I wont be running one of those ever. That piece of Extractor in my cheek and eyebow was enough for me.
Russel

 

[Dave Marshall]

After finally getting together with my gunsmith and a bore cam, unfortunately there was no definitive evidence of what caused the case failure. There were no obvious issues with the chamber or bore.

I’m still inclined to believe there was some issue with the chamber dimensions, but will never really be able to determine for sure.

 

[Zero333]

Probably excessive headspace was the cause.

I say this because you didn't measure the base to datum of all the new brass, so you really don't know if it was all within spec.

 

[243winxb]

into a carbon ring.

The case mouths measure .268 for the intact case and .272 for the ruptured case. The neck diameter also increases to .274 as it is measured toward the shoulder. Based on these measurements, it seems the case mouth was somehow constricted causing a drastic pressure increase in the cases.

The neck of the chamber was reamed with a .276 neck reamer for more reliable feeding.

I had the chamber re-cut with a larger 6xcII reamer. This pushed the shoulder about .002-.003 forward. Headspace difference with the new chamber and new brass is .007-.010. It’s more than most would like, but not outside of SAMMI allowances.

When the shoulder was cut slightly deeper, it allowed the case mouth of the new, untrimmed and longer brass to be pushed into the tight area of carbon buildup. This would cause a huge spike in pressure when fired.

Seems the neck got pinched in the small or short chamber.

 

[Dave Marshall]

Probably excessive headspace was the cause.

I say this because you didn't measure the base to datum of all the new brass, so you really don't know if it was all within spec.

The headspace was fine. This was premium brass there’s not enough variance in the new brass to even bother measuring it.

 

[Dave Marshall]

Seems the neck got pinched in the small or short chamber.

That’s my best guess as well. Unfortunately, there were no obvious problems visible in the chamber.

 

[Zero333]

That’s my best guess as well. Unfortunately, there were no obvious problems visible in the chamber.

I say you won this one because your face didn't get any uglier from the mishap.

Your adventure is a serious reminder ( to those of us who are too arrogant to wear safety glasses ) to wear our safety glasses !!! Call me dumb, but I don't wear my safety glasses most of the time when casually shooting rifles, but for whatever reason I always use them when shooting pistols.

It's not a perfect world and all we can do is manage the risks.

On the other hand, life would be stupid boring with out some risks !

 

[ckaberna]

Can’t you cut another chamber with the exact reamer and try to replicate the incident (of course with inert rounds) to determine where the problem is?
Neck/throat?

 

[johnfred1965]

Just a spitball here: Is is possible for the flash hole to be big enough to allow detonation of the powder at the base of the case rather than ignition?