someone explain this! case dent after firing

[RU shooter]

Riddle me this Batman, I was out shooting my M39 this morning and using my normal favorite load of a lee 312-185 w/gc over 30 grs of pulled Russian powder, I fired about 15 so far and then out pops this case with a huge dent from the base of the neck down approx 3/4-1" and atleast 3/16 deep???? Nothing was out of the norm. and even the boolit went into the same group .
So I check the chamber real good ,nothing unusual ! fired about 2 more rds. without incedent and I get the second dented case,What is going on here???? Same exact place same exact size same shape !!!! I had fired about 30 rds of a different load before shooting this load and then fired about 20 J word rds after the dented case load and nothing unusual at all! I have had this rifle for about 5 yrs now and fired countless rds without a single issue! What would cause this? how do I prevent it from happening again? This is about the strangest thing I have seen in my 20 some yrs of reloading.

[JSH]

I have seen this before. You are shooting the boolit so fast that the vacum sucks the brass in.
Sorry had to do that. Told a fellow that a few years ago and had him believeing it,lol.
How many times have those cases been reloaded. I would say that the necks are getting hard and letting the gas blow back down between the chamber and the brass. May be time to anneal.
But on the flip side, I did see one fellow have this happen after annealing. He annealed way to low and didn't do the neck. Which in turn made it worse as the case body was to soft and the neck to hard.


Jeff

[SharpsShooter]

I have seen this before. You are shooting the boolit so fast that the vacum sucks the brass in.
Sorry had to do that. Told a fellow that a few years ago and had him believeing it,lol.
How many times have those cases been reloaded. I would say that the necks are getting hard and letting the gas blow back down between the chamber and the brass. May be time to anneal.
But on the flip side, I did see one fellow have this happen after annealing. He annealed way to low and didn't do the neck. Which in turn made it worse as the case body was to soft and the neck to hard.


Jeff

Yep, the crimp is so tight the boolit tries to drag the brass along

Time to anneal or replace the brass

SS

[TAWILDCATT]

NORMALY i WOULD THINK IT WAS MADE WHEN THE CASE WAS EJECTED.however the dent is too rounded it looks like the gas is getting between the case and chamber.however did you get gas in your face or see it out of breach.did you check your loaded shells. after that who knows?

[S.R.Custom]

... I would say that the necks are getting hard and letting the gas blow back down between the chamber and the brass.

Ummm... for that to happen there would have to be more pressure outside the case than inside...

[Bent Ramrod]

I believe it's a kind of reverse obturation; if I recall it also happens occasionally in well casings when an explosive is used to cut the casing to let in the oil at multiple levels in the well. Some of the gas pressure gets in between the rock and the casing (or between the chamber wall and the brass shell) and as the pressure drops inside the borehole (or the bore), the trapped gas pressure pushes the metal into the low-pressure central area. I don't think this is anywhere near the working pressure of the powder, but it doesn't take a great deal to push unsupported thin metal, forming a dent.

I would guess that your load is lower-pressure, not allowing the case to totally expand against the chamber wall. If the case is work-hardening, as JSH suggested, this could aggravate the condition as well.

[SharpsShooter]

Super,

If the work hardened case neck does not fully expand to seal the chamber then pressure is applied to the exterior shoulder of the case following the boolits departure from the case and continues until the boolit exits the bore. The case neck is often work hardened due to sizing and expansion during firing, leaving the shoulder and case body relatively soft and in some cases (pardon the pun) vulnerable to pressure deformities from gas blow by from the unsealed neck. Often the shooter gets a face full of gas and small particles or oil from the bolt itself.

SS

[RU shooter]

The brass I was using was all reloaded 6-7 times and the theroy of gas getting between the case and chamber does seem logical except if its because of too low of chamber pressure in that specific load why didnt I have any issues in the other loads I shot earlier 8 and 12 grs of 231 with a 155 gr boolit (alot less pressure there) And no I didnt recall any gas leakage from the bolt I did with the last load I shot 20 grs of 2400 and Jwords but only on one or two rds and very mild amount.

[trk]

It's gas leaking back around the neck between the case and the chamber. The case empties of pressure once the bullet passes out the barrel the case caves.

Some folks prevent this, and getting the soot from blowby, by leaving just a LITTLE flair outward on the neck. (Doesn't help much on crimping.)

I would also agree that if the brass was softer it would seal better. I've only had it happen once - and then I switched to neck sizing only half way down the neck.

[felix]

231 is a fast powder. ... felix

[BruceB]

When my wife's beautiful Browning Mauser blew up, the recovered case exhibited a shoulder collapse much like the one illustrated. It was BIG!

It is my firm belief that there was a HANGFIRE, because the bullet traveled only about four inches from the case, and stopped.

I envision this in the case of Karen's Browning:

-the primer fires, and a part of the load ignites, driving the bullet out of the case.

-the bullet stops as the powder load fizzles, and since the pressure is NOT sufficient to seal the chamber, some gas comes back outside the case and collapses part of the case. Note that this now also replicates the well-known circumstances for SEE, as well.....namely, a decreased load of slow-burning powder in a much-enlarged volume.

-the powder finally gets burning, and the pressure behind the bullet, both inside the case wall and outside the case wall are identical, allowing the collapsed area to remain in its collapsed state.

-in my wife's rifle, the pressure generated was sufficient to DESTROY the rifle, even while the case collapse remained.

I strongly suspect that the rifle in the current thread has been hangfiring, even if the hangfires are too short to be noticed. I would view those collapses as a very serious WARNING to change the load rather drastically. For openers, I'd use a dacron filler, at least, but also I'd be inclined to use a powder load of known characteristics (meaning, published loads with U.S. powders) and see how well they work in the rifle.

I doubt that work hardening has anything to do with this event. Even work-hardened brass isn't all that hard, and in my wife's rifle, the case was once-fired only, meaning there was no possible effect from work-hardening.

[44man]

I can't disagree with a thing Bruce says! Sounds like one step away from a blown rifle. I would use the remainder of the powder for fertilizer.

[1hole]

Bottom line is your chamber pressure is too low to expand the cases and seal the chamber properly.

It may be agrivated by work-hardened necks or a different lot of powder or ..... other mystical factors. But that doesn't change the fact that your pressure to simply too low to effect a seal.

[JSH]

Fellows, what I was going on was experiance I had. Almost the same thing, but the dents were not quite as bad. Now that Bruce mentioned the hang fire, I remember seeing that once before on a rifle at the club a gent was shooting. I don't recall make or caliber, but the dent sure looked like the one in the pic.

What I had experiance with was a 7BR and a 7TCU. Sooting and so forth all the way back to the action face and bolt face. Not a bit of smoke came out around the action. I annealed all of the cases and problem solved. These were by no means squib loads or light. Then the 6 TCU started showing the same sooting and it had a large dent like a lube dent in it. I susupect that it was because of the case shape as to how the dent was made. Annealed those and problem solved.
The TCU cases had all been neck sized only and had been shot a lot.
I would also suspect the long gradual type taper pf the 7.62x54 would lend itself to "blowby" a lot more so than other designs. I know I know what some of you are thinking. But the 7.62x54 was never intended to be reloaded as were most early designs. I guess some may have just been luckier than others.
Jeff

[DLCTEX]

The three other times I have seen and read of the case denting illustrated in the pics. 231 powder was involved. Coincedence? Light loads not generating enough pressure until secondary ignition is the best guess I've heard. Dale

[texas tenring]

I had the same thing happen with a 91/30 shooting .308 J-bullets and a medium speed powder. It is the neck not expanding to make a seal and letting back presure dent your case. In my opinoin the faster burning powders give you more bang! so to speak and won't give you that problem with a light load. A medium or slow burning powder in a light load won't give enough chamber presure to make the seal. I've not had any problems with cast boolits and 2400 powder.

[RU shooter]

The three other times I have seen and read of the case denting illustrated in the pics. 231 powder was involved. Coincedence? Light loads not generating enough pressure until secondary ignition is the best guess I've heard. Dale

This did not happen from my 231 loads these are from a load of 30 grs of pulled powder from russian 54R rds. the burn rate of this powder is between 4895 and 3031 and have used it with good results in other calibers using 3031 data. The avg load from a 150gr ball rd is 47.5 grs .
My load manual states a 44gr starting load for a 180 gr jacketed bullet with 3031,So I am loading these at 2/3 for a 185 gr cast boolit . As stated I have shot atleast 100 rds of this same load with no problems. As far as hangfires I have had them with milsurp ammo and know what they feel/sound like , But these in no way acted like a hangfire. I looked over my other fired cases and there was no sign of sooty necks or cases. I'm not trying to dispell anyones theory just stating more facts So I can get a better grasp on what caused this

[danski26]

I've never seen anything like this before. All the theroies sound possible.

My question with all of them is, how is it the exact same dent in the exact same place?

Hangfire or hardened neck wouldn't the dent be random in size and location, driven by the variables in case construction, hardness level, position of bullet in the bore "mentioned in hangfire situation", exact position of the powder in the case and so on?

[Linstrum]

I would stop using the rifle and get the chamber Magnifluxed or dye-tested before resuming shooting since a crack in the chamber wall will cause this kind of phenomenon. A crack that starts from the inside at the chamber wall sometimes won’t go all the way through to the outside at first, and if the crack communicates with any gas pressure before the brass seals during firing the crack will harbor gas under pressure like a small air tank. When the pressure in the barrel drops the gas is released back out from the crack and its pressure bulges the case inward.
I hope there is some other explanation, but I have read about this before some time back in one of the gun rags many decades ago, probably in the 1950s. You can get fluorescent dye and magnetic type crack testing kits online now, but I don’t know what they cost. I'd check it out because we are not very often given a danger warning like this that a violent catastrophic failure is about to occur!
Good luck, I hope it is not the worst-case scenario.

[dromia]

This was quiet a common occurence with FNM 6.5x55 Swedish factory ammo, percieved wisdom at the time was slow pressure build combined with brittle brass allowed gas to escape past the neck but the rear of the case had sealed the chamber. No gas escape and the round felt and shot as normal, these cases also universally sooty with the odd shoulder collapse.