Strange dent in fired case

[Mike Lum]

Next day at Ed's range they have the big blue rain barrels full of water by the bench area labelled "Duds" after a staff member messed his pants finding that casing on the ground.

 

[bozo699]

In your case Wayne your dents are from over lubing your cases.

Not hardly Ed
Wayne

 

[tommeboy]

If you get a dent like that after firing its usually caused by a “Secondary explosion effect”


Low neck tension with a big jump on a cool day might do it, but usually its from an undercharge with slow powders.

No such thing as a SEE in a firearm. Only happens in Naval guns. It is called a high pressure excursion by all the powder and bullet makers.

What happened here is nowhere near that. It can happen a number of ways. Very low pressure not sealing the neck, then the gas is trapped by the now sealed base of the cartridge and has nowhere to go. Throat very worn and trying to seat bullet out to touch and the bullet leaves the case before it reaches the rifling is a huge contributor of this.

 

[Canadian bushman]

No such thing as a SEE in a firearm.

You sure about that? There sure is a whole lot of information about such events and a good bit of warnings against such events, from powder mfg’s, for the situation to be false?

Also. How does the head of a case seal before the neck of a case? The neck is a good bit thinner and way softer?

 

[MagnumManiac]

This is NOT caused by the bullet leaving the case and a gap between case mouth AND bullet!
If that was the case, every Weatherby on the planet would dent every case upon firing.
It is simply a low pressure instance that allows gas to pass by the neck. Often it happens with a middle load as well and hard brass necks. If the powder is slow to ignite, which also happens from time to time, then the dent is evident, if it’s really slow to ignite and doesn’t burn correctly, the dent and soot can be seen all over the case right down to the case head.

Cheers.

 

[dkhunt14]

I have seen it happen with a wet chamber. Like after cleaning and not getting it dry enough. Matt

 

[Laurie]

It's a 'powder dent', otherwise known as a 'gas dent' caused by trapped gas between the case body and chamber wall. Ignition is such that the case either doesn't obturate fully, or if it does too slowly, in either case letting some gas escape backwards from the mouth and become trapped.

IME, you see this with too low starting loads and/or poor powder characteristics for the application, and /or an insufficiently vigorous primer for a slow-burning powder. Some cartridges are more prone to it than others - 6.5X55mm in Swedish Mauser low-pressure loads are very prone to it and I've seen it many times with this cartridge.

FNM, a long defunct Portuguese ammunition manufacturer produced a range of FMJ 'target' ammo for a range of mostly vintage military calibres many years ago - 303 British, 6.5X55mm, 7.5mm Swiss and French Mas versions, 30-06 M2, 6.5X54 Mannlicher plus the ubiquitous 308 and 223 M180 and M193 knock-offs. All cartridges used a fine-grained ball type powder, pretty fast burning and with lots of air-space and it seems the one grade only irrespective of the cartridge. This combination definitely didn't suit some of the range, especially the 6.5X55 where two or three gas-dented examples usually appeared per 100 rounds especially in cold weather. Badly sooted but not dented cases caused by poor obturation were even more common and seen in several cartridge models.

You sure about that? There sure is a whole lot of information about such events and a good bit of warnings against such events, from powder mfg’s, for the situation to be false?

The evidence and warnings refer to use of slow burning magnum powders in large case capacity designs where it is important to stick to the recommended starting load as a minimum charge weight. For reasons that have been much speculated upon but never conclusively bottomed, a too low charge in such a situation can produce a pressure wave or maybe a sudden detonation rather than progressive charge-burn producing dangerous gun-wrecking pressures. You see warnings in the load table for some magnum cartridges that charges weighing less than the listed starting loads must never be used and some powders have starting loads only slightly below maximum. (Another reason for these instructions may also be that lower loads will produce inconsistent pressures/velocities.)

 

[Kevin Taylor]

This is NOT caused by the bullet leaving the case and a gap between case mouth AND bullet!
If that was the case, every Weatherby on the planet would dent every case upon firing.
It is simply a low pressure instance that allows gas to pass by the neck. Often it happens with a middle load as well and hard brass necks. If the powder is slow to ignite, which also happens from time to time, then the dent is evident, if it’s really slow to ignite and doesn’t burn correctly, the dent and soot can be seen all over the case right down to the case head.

Cheers.

This.

 

[davidjoe]

Not doubting the low pressure answer, it’s just hard to envision. To dent the case inward, pressure between the chamber wall and brass had to be greater than pressure inside the case pushing outward.

It’s kind of like envisioning dropping a running water hose into a swimming pool then finding later that the pool’s water pressure pinched the water hose so tight that the water stopped running.

I am supposing the better analogy is thrusting a cardboard tube into the wind while driving fast. If it’s manipulated just right, the wind will collapse the edge into a wedge or point then crush the point. This assumes much of the powder gets expelled and at some point there is higher pressure mid barrel than in the case. If the origin of pressure, (the cartridge) always had the most pressure of any area inside the barrel, then I would be doubting that low pressure explanation.

One thing no one addressed is why the neck remained perfectly circular.

 

[dellet]

Not doubting the low pressure answer, it’s just hard to envision. To dent the case inward, pressure between the chamber wall and brass had to be greater than pressure inside the case pushing outward.

It’s kind of like dropping a running water hose into a swimming pool then finding later that the pool’s water pressure pinched the water hose so tight that the water stopped running.

I am supposing the better analogy is thrusting a cardboard tube into the wind while driving fast. If it’s not held just right, the wind will collapse the edge into a point then crush the point. This assumes all the powder gets expelled and at some point there is higher pressure mid barrel than in the case. If the origin of pressure, (the cartridge) always had the most pressure of any area inside the barrel, then I would be doubting that low pressure explanation.

One thing no one addressed is why the neck remained perfectly circular.

The air between the case and chamber is trapped when the case finally seals the chamber. When the chamber pressures decreases to less than trapped air pressure, is when the deformation can begin. At bullet exit there is no internal case pressure, only the trapped air pressure.

The neck only remains circular, if the air trapped is below the neck, in the body. In a straight wall case you often have a dent that also collapses the neck.

 

[Canadian bushman]

I dont care what you call it or refuse to call it. I dont care what you think is happening inside the chamber, in which no one can see, and no one knows for sure.

If you google Secondary Explosion Effect you will get all the info you need on exactly this. Even if you refuse to concede to this name.

This is a hugely common scenario when loading under starting loads with medium to slow burning powders. Extremely common in magnum pistol cases.

Bring the charge weight up and it will go away.

 

[davidjoe]

I follow the expanding trapped gas rationale. A tiny trapped kernel can also make a huge dent but it usually has a more discernible origin. I have wondered why the dent is so much larger than the kernel causing it.

Separately I have seen something that resembles a secondary explosion demonstrated in slow motion via high speed camera on a clear acrylic or PVC pipe launching potatoes. They’ve even captured cylinder head combustion this way to study burn efficiency.

 

[XTR]

This is NOT caused by the bullet leaving the case and a gap between case mouth AND bullet!
If that was the case, every Weatherby on the planet would dent every case upon firing.
It is simply a low pressure instance that allows gas to pass by the neck. Often it happens with a middle load as well and hard brass necks. If the powder is slow to ignite, which also happens from time to time, then the dent is evident, if it’s really slow to ignite and doesn’t burn correctly, the dent and soot can be seen all over the case right down to the case head.

Cheers.

It can be, if the bullet is seated far enough out of the case.

I had a load for the 200-20x bullets in a very long throated barrel. I had about 25 thou engagement on the bearing surface. Bullets jumped about 15thou. Shot like a laser though. Finally took it off after about 3500 rounds. (wasn't planned that way, just turned out that a barrel throated for 200Hybrids shot the 200-20x really well in that configuration)

In any case, I was using those bullets as a known load for comparison to run pressure tests with my pressure trace in a barrel that I had throated even longer for 215s. That barrel was throated 25 thou longer. I noticed after about two shots that I was getting smoke from the closed bolt and the cases were sooted all the way to the boltface.

So, in the absence of enough neck tension/engagement and enough freebore to allow the bullet to get out of the neck it can happen.

 

[dellet]

It really has nothing to do with the bullet leaving the case, other than the pressure it might release from the case, into the chamber area.

If the case never expands enough to seal the chamber, you get soot below the neck and shoulder. When it seals late trapping gas, you can get a dent or collapsed case.

The way this is prevented in low pressure loads, or gasp, breech seated loads where the bullet is never in the case, is to flare the neck so that it seals the chamber artificially.

The dented case can be created/prevented on demand with low pressure loads by running zero headspace or a slight crush on a bottle neck case. Or by flaring the lip on the neck of a case that headspaces off the rim or straight wall case.

 

[Laurie]

The way this is prevented in low pressure loads, or gasp, breech seated loads where the bullet is never in the case, is to flare the neck so that it seals the chamber artificially.

Interesting! A schuetzen rifle trick?

 

[dellet]

Schuetzen is probably best example for breech seating or muzzle loading a cartridge rifle. You’ve probably seen that.

Antiques in general that anything over 10-12,000 psi that might be stressing the actions

I have found low pressure fixed smokeless loads in black powder cartridge rifles benefit from the flair also. Smokeless builds pressure too slow to expand the neck seal like black powder would. Add the flair or bell like loading pistol and leave it.

If bullet, case and chamber match well then seating the bullet with thumb pressure is normal without sizing. The flair seals the gas from blowing back and is flattened out when fired so the only “sizing” is the flair.

Load the bullet long and soft seat in the lands.

That thinking transferred over to sub sonic shooting with a bushing die and only sizing part of the neck. Leave the base of the neck and shoulder alone. Less gas in the face. Works well in a blackout AR.

Doesn’t work too well on a tube fed lever rifle, but very nice in single shots.

 

[M-61]

sudden detonation rather than progressive charge-burn producing dangerous gun-wrecking pressures.

Parker Ackley wrote and worked on this for a very long time.
From what I have read over the years large cases with below minimum loads may be the culprit. The problem seems to be you can't replicate this with any degree of certainty. However the warnings are numerous and quite clear.

 

[243winxb]

Nothing new- from my 1970 Speer Manual Number 8 that i learned to load with. Page 57.

 

[tvaught]

Nothing new- from my 1970 Speer Manual Number 8 that i learned to load with. Page 57. View attachment 1205779 View attachment 1205780

Awesome reply! Thank you for sharing this

 

[dskogman]

Nothing new- from my 1970 Speer Manual Number 8 that i learned to load with. Page 57. View attachment 1205779 View attachment 1205780

Good find!
After doing more testing and shooting a 6x20 match over the weekend it appears that increased neck tension fixed the issue.
The load is 61.5 gn of H4350 with a 215 Berger hybrid so it’s close to a max load. It has a .250 FB and jumping .020. It didn’t happen often (twice) and when it did the gun was cold. Thanks for all the reply’s it was a great help!