I saw a case today that had a head seperation. It had dents just below the shoulder that sorta resembled case dents from resizing with too much lube. I think there were three or four. They ran north and south and about 25% of the length of the case. What cause this? I would think even with the case head seperation the pressure would be suffcient to blow the case against the walls of the chamber as normal. Not sure what caused this. It was a usual load and several of a bunch fired. There was no blowback and it sounded normal and the bullet even landed in the group. I fiqure that case should have been culled with the case head deal but what about the dents?

What kind of gun/cartridge?

it wouldn't by any chance have been a 7mmRUM would it?

It's my 8rem mag. It's a m700 action recently rebarreled by Krieger. I don't think it's related to the chamber. I toss the brass for the gun every three firings regardless, but that piece may have slipped by. I can understand the case head, not the imploding type dents.

Jack,I've sent you an E Mail on this only because I don't know how to post photos. If you know how to post them I have no objection.It's something that has not been adequately answered as far as I know. roger

Dempsey,

Was it the first shot after having cleaned the rifle? Perhaps it was due to excess solvent or oil not cleaned out of the chamber. Hope you figure it out, Bruce

Roger
Pretty much the same thing but a little longer. I left the rifle and case with the Gunsmith and will check back Monday.

This was about the twelve shot of the day. I'll report what my Smith thinks it is, he's pretty sharp and been doing it for a long time so hopefully he'll know.

Here's the picture Roger sent me.

I spoke with the Smith today and his explanation seems pretty straight forward. Basically he told me when the case head let go gases creeped forward between the case and the chamber, the neck still sealing the chamber, and hence the dents. Makes sense, basically I missed that cull and suffered the embarrassment The dent such as Rogers was explained as generally being the result of a low pressure situation, the neck doesn't seal the chamber and a little gas blowback can cause such dents. These aren't quotes but I think I got it right. In any case no harm done other than the my error.

The explanation from your GS is one I've heard before.In order for that to happen the gas blowing back on the outside of the case would have to be at a greater pressure than the gas on the inside of the case.Unless we write a change notice to the laws of physics the brass isn't going to move if the pressures in and out are equal. You would think you also would see sooty flow marks. Others on this forum in the past say they also witnessed this phanominum.It would be interesting to revisit this scenario and maybe evaluate all the evidence.Maybe we can establish some commonality. roger

Roger, the physics of it would give me a headache if I attempted to speak on something I don't know. But just guessing it doesn't seem beyond reason that a point may exist where the pressure on the inside of the case falls below that of the outside. I'm just glad no damage was done. I moved last fall and lost track of all sorts of things. I tossed all my old brass and I'm starting new. Thanks for sending that pic.

I'm sure that at some time the pressure in the dent area was larger than the pressure inside the case. The maddening thing is how. nice talking to you roger

we have seen this with several 7mmRUM's with handloads using slow powders (retumbo, H870, etc) The GS and I decided that the oversized necks in factory chambers, coupled with hardened brass (even first firing rem brass) and the slow burn rate of the powder caused that gas blowback. Once the neck does seal,..that trapped gas colapses the case. That was why I asked about the 7rum. I havn't seen it in my 300RUM with retumbo and 216gr pills yet,..but the reamer and barrel were match grade,.so the chamber will no doubt be far tighter and hence the problem non-existant. Currently, the gunsmith has a Mcmillan rifle in the shop in 7mmRUM which is doing the same thing. Mcmillan checked and said it is fine,..but has agreed to let the GS see what he can find,..and he suspects an oversized neck as well.

Now your explanation makes sense. In my case I was using 5020 powder and I'm sure the neck wasn't sealing until after the bullet was slightly up the barrel allowing gas to leak a little past the shoulder. The increasing pressure finally sealed the neck trapping the gas in a small pocket. As the internal pressure began to drop the trapped gas at now higher pressure formed the dent and leaked out into the barrel. I think this is what you were also saying if I read you right. By George I think you've got it. Great thread. Thank you Just C Roger