Oversize flash hole/case rupture

[Dave Marshall]

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.

 

[Sniper338]

Im scared to say this, and its not to put a blame anywhere, but one would think a gun smith rechambering a barrel would look in it with a scope, or cut off alittle more of the chamber to re chamber more of a new part of the barrel. No clue here but it sounds like, and Ive heard it said before, when rechambering cut off some of the barrel then rechamber? This seems like it would have taken care of this phenomenon thay ended horribly.

 

[Dave Marshall]

Im scared to say this, and its not to put a blame anywhere, but one would think a gun smith rechambering a barrel would look in it with a scope, or cut off alittle more of the chamber to re chamber more of a new part of the barrel. No clue here but it sounds like, and Ive heard it said before, when rechambering cut off some of the barrel then rechamber? This seems like it would have taken care of this phenomenon thay ended horribly.

I don't blame the gunsmith. He's a good friend and extremely thorough craftsman. We still don't know for sure what caused it, but this is the leading theory so far.

You are correct that a normal setting back of the barrel would have eliminated any problem like this. This was simply done as a test of the new brass for function. A full setback would have been done at a later time. It may end up as simply a lesson learned the hard way.

 

[Sniper338]

I don't blame the gunsmith. He's a good friend and extremely thorough craftsman. We still don't know for sure what caused it, but this is the leading theory so far.

You are correct that a normal setting back of the barrel would have eliminated any problem like this. This was simply done as a test of the new brass for function. A full setback would have been done at a later time. It may end up as simply a lesson learned the hard way.

It will be interesting to see the barrel cut in half. That should complete the investigation.

 

[Ggmac]

I don't blame the gunsmith. He's a good friend and extremely thorough craftsman. We still don't know for sure what caused it, but this is the leading theory so far.

You are correct that a normal setting back of the barrel would have eliminated any problem like this. This was simply done as a test of the new brass for function. A full setback would have been done at a later time. It may end up as simply a lesson learned the hard way.

OK , after many years of setting barrels back . I usually set the chamber deeper AT LEAST the length of the neck . This is because it's a gamble the 2 dif Reamers used would have identical neck dimensions . It's possible but not likely , considering original smiths set up ( runout ) and the current smith . Even if both Reamers have identical neck dimensions , shooting wear ( erosion ) from the original chambered neck would leave a small but possible dangerous larger area just in front of the old neck .
I'm glad your OK and please , EVERYONE WEAR GLASSES !

 

[Dusty Stevens]

If the gunsmith inspecting it is the one that chambered it then its not a faulty chamber.

 

[Dave Marshall]

If the gunsmith inspecting it is the one that chambered it then its not a faulty chamber.

Did you miss the part where I said he’s a friend and teammate?

I’m very sorry you don’t trust your friends.......

 

[Mulligan]

If the gunsmith inspecting it is the one that chambered it then its not a faulty chamber.

Are you saying that every gunsmith is without integrity and cannot admit they may have made a mistake????
CW

 

[Dusty Stevens]

Not at all. Im pretty interested in the cause here though

 

[Dave Marshall]

Not at all. Im pretty interested in the cause here though

I won’t have any more info on the barrel/chamber for at least a week. I’ll try to remember to update the thread.

 

[Dusty Stevens]

I won’t have any more info on the barrel/chamber for at least a week. I’ll try to remember to update the thread.

Id love to see the sectioned chamber/throat area. Thats where i suspect the issue is

 

[fyrewall]

After digging out the front part of the blown out brass will an identical loaded round easily slide into the chamber and headspace normally?

I have seen stuff sort of like this from incorrect powder loads - like H4895 instead of H4831. In that situation the primer pocket looked egg shaped about .27 caliber and the barrel needed to be unscrewed from the receiver, brass head smeared all over bolt face and the extractor was busted. Your primer pocket looks round but the flash hole looks huge.

A look at the front part of the brass shows almost a smooth cut as if the brass was sheared off except for an area at about 12:30 - 2:30 o'clock. A side view shows the brass head that was extruded out from the side and the brass extractor groove also has been partially filled. Apparently the extractor was a secondary missile.

Are the case walls of normal thickness down to the bottom inside of the brass? Another thing to look at would be the degree of work hardening that occurred during the case manufacturing process - possibly the base of the brass case was annealed. Sectioning the front part of the case might show some case forming defect. The diameter of the rim looks normal. That oversize flash hole might be part of an ongoing major defect.

The innards of the front portion of that case remaining in the barrel look a clean as my pistol rounds loaded with Clays powder. Was the inside of the case remaining in the barrel cleaned for some reason?

I doubt that there are any issues from carbon rings and gross chamber anomalies. Checking out things with a bore scope might be useful.

After signing off I will toss a bunch of olde brass.

 

[Doug30racer]

Just a thought, was there ever an unaccounted for primer missing. Years ago I had a neighbor loading his own shells at my place, an had a near rifle explosion. Didn't hurt the barrel, but it wrecked the bolt. Later after checking things out, I found in the container of primed brass that 2 were missing primers. I check out way and the pockets would not hold them any more. After digging I found one but not the other. So I have concluded that the shell the wreck things was loaded with a primer random in the case with the powder. I think we used h4831 so even with the case full it would not do that. Just a thought. He is way more fussy about the brass he uses now

 

[Dave Marshall]

After digging out the front part of the blown out brass will an identical loaded round easily slide into the chamber and headspace normally?

I have seen stuff sort of like this from incorrect powder loads - like H4895 instead of H4831. In that situation the primer pocket looked egg shaped about .27 caliber and the barrel needed to be unscrewed from the receiver, brass head smeared all over bolt face and the extractor was busted. Your primer pocket looks round but the flash hole looks huge.

A look at the front part of the brass shows almost a smooth cut as if the brass was sheared off except for an area at about 12:30 - 2:30 o'clock. A side view shows the brass head that was extruded out from the side and the brass extractor groove also has been partially filled. Apparently the extractor was a secondary missile.

Are the case walls of normal thickness down to the bottom inside of the brass? Another thing to look at would be the degree of work hardening that occurred during the case manufacturing process - possibly the base of the brass case was annealed. Sectioning the front part of the case might show some case forming defect. The diameter of the rim looks normal. That oversize flash hole might be part of an ongoing major defect.

The innards of the front portion of that case remaining in the barrel look a clean as my pistol rounds loaded with Clays powder. Was the inside of the case remaining in the barrel cleaned for some reason?

I doubt that there are any issues from carbon rings and gross chamber anomalies. Checking out things with a bore scope might be useful.

After signing off I will toss a bunch of olde brass.

The shoulder of the chamber was damaged when I removed to case body. Chambering another round wouldn’t tell us anything due to the damage.

No way this was a powder issue. I loaded 100 rounds at once. 40 in the box had already been fired in the older chamber.

The pressure in the chamber rose too high, too fast. Most likely due to the case mouth being constricted. The brass began to flow/extrude into the ejector plunger and through the extractor groove. When that didn’t relieve the pressure fast enough, the case ruptured at the base where it is not supported by the chamber walls.

The inside looks clean because I tried using a large chamber brush and chamber mop to extract the case body.

 

[Dave Marshall]

Just a thought, was there ever an unaccounted for primer missing. Years ago I had a neighbor loading his own shells at my place, an had a near rifle explosion. Didn't hurt the barrel, but it wrecked the bolt. Later after checking things out, I found in the container of primed brass that 2 were missing primers. I check out way and the pockets would not hold them any more. After digging I found one but not the other. So I have concluded that the shell the wreck things was loaded with a primer random in the case with the powder. I think we used h4831 so even with the case full it would not do that. Just a thought. He is way more fussy about the brass he uses now

Not a primer problem. All dropped primers are accounted for before I continue priming more cases.

 

[joshb]

What is the distance between the closed bolt face and the barrel?

 

[mrjdasr]

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.

lets see if I understand this, the new reamer bumped the shoulder forward .002 but left the overall length of the chamber the same as before??
then you used new brass that had an overall length that exceded the length of the new chamber
how else could your new reloads reach the carbon ring
if the new reamer increased the overall length of the chamber, it should have also removed the carbon ring
I don't want to make the same mistake you experienced
think ill stik to cutting the old chambers all the way off

 

[Dave Marshall]

What is the distance between the closed bolt face and the barrel?

Probably less than .005. This dimension was not changed from the initial fitting by the re-chambering.

 

[Dave Marshall]

lets see if I understand this, the new reamer bumped the shoulder forward .002 but left the overall length of the chamber the same as before??
then you used new brass that had an overall length that exceded the length of the new chamber
how else could your new reloads reach the carbon ring
if the new reamer increased the overall length of the chamber, it should have also removed the carbon ring
I don't want to make the same mistake you experienced
think ill stik to cutting the old chambers all the way off

You are correct that the new reamer should have cut the neck at the case mouth to the full diameter and length. It may or may not have. We don’t know yet. The differences between the old reamer and the new one are primarily at .02 from the base of the case. The neck, shoulder, and upper body dimensions are nearly identical.

My brass length is within specification, but the the older brass is definitely a little shorter than the new brass.

The reamer itself could be ground wrong too, although unlikely. I’ll also need to inspect it. It is brand new. This was the first use.

It’s going to be a long week waiting for the smith to return from vacation.....

 

[Dave Marshall]

An inspection of that chamber with a bore scope might provide an answer. I am trying to get past, "The shoulder of the chamber was damaged when I removed to (the) case body". Any views of the headless case including case shoulder? This might show a bashed up case shoulder. We still have not seen the remains of the headless case. Peering into the inside of the chamber might not provide very good assessment of chamber damage but dropping an identical sized round from the remaining ones in the lot might - would it fit. Again, checking out the remains of the headless case might show the nature of any chamber damage.

There seems to be some confusion regarding the inside dimensions of the chamber - would dropping a correctly sized round to the chamber indicate an abnormally tight neck that would prevent the round from being easily chambered. Would inspection of the recovered headless brass and inspection of the neck show an such a tight neck that a .243 diameter bullet would not be easily pushed into it. Possibly, the rechamber job was done making the neck of the chamber diameter .243 instead of about .275 or so. Would dropping a .243 bullet into the chamber show any thing if the .243 bullet would not easily slide down the chamber and engage the rifling.

I have always thought of the possibility of the brass neck portion becoming fused onto a bullet and separating from the rest of the case, thus increasing the bullet diameter by .030 or so - the 6mm bullet would be a .27 and pressures would go crazy. I have had brass necks separate from the rest of the brass but they always remained in the chamber. Pulling the remaining bullets from that lot was difficult as they seemed to be cold welded in place and the neck portion was really jammed in to the chamber.

Any chance of annealing causing a soft head? That huge flash hole gives me problems.

I used a carbide drill bit to bite into the case body to extract it. I was trying to be gentle, but managed to damage the shoulder area of the chamber with the drill bit. Trying another round into the chamber will not headspace correctly due to the damage. It would not reproduce the problem, if the is there. Dropping a bullet in would also not show us anything.

I didn’t take any photos of the case body, and it was sent back to the manufacturer. The cases were tested for hardness and are within specification.