Did you measured everything before loading and know for certain that case was the same dimensions as the other cases?

My first thoughts: Headspace - who says the sized (unfired) case fits the chamber correctly. The shouler may have been moved back too far in relation to the chamber. Best to do a chamber cast and measure it.

Hoot:

Do a chamber cast and check to see if the chamber is a bit long. What it sounds like to me is that the firing pin drove the case forward and the primer backed out when the cartridge fired. That would explain the backed out primer and missing the extractor.

Just a thought.

Rem

Primers are really much more powerful than we give them credit for. In your case, the PRIMER drove the case forward when it fired (not the firing pin...) and set the shoulder of the case back. Yes, the primer has more than enough force to do this. It fires and drives the case forward and the primer rearward, leaving the primer protruding from the base of the case and the case driven forward. On a "normal" full power cartridge, the combustion of the powder and the pressure created counterbalance this by pushing the case rearward and holding it against the bolt face. That doesn't happen with low pressure rounds. I've had this happen countless times with rimless cartridges like the 30-06, 35 Rem, 308 Win, etc... If you don't believe me, simply prime an empty case, chamber it, and fire the primer and see what happens. If you think that the firing pin is driving the case forward, simply take a fired case and fill the primer indent in with solder, chamber the case and drop the firing pin on it and compare what the result is with the case where you fired the primer. (I have had looooooooong discussions on other boards about this, and it is hard to get some people to understand that it is the primer that does this, not the firing pin...)

Now, the question is how to stop this from happening while keeping the pressures and velocity low. The solution that I use, is to switch to a faster powder. I typically use Unique. In the 8mm, I would guess that between 10 and 13 grains of Unique should do what you want. I don't use fillers, but if you really feel the need, you could use say a 1/4 sheet of toilet paper wadded up and pushed down on top of the powder, but again, I don't think that fillers are needed. You might also try switching to a less powerful primer like a large pistol primer. You could also up your powder charge to try to generate enough pressure to hold the case back against the bolt face.

You've also just discovered why rimmed cases are popular for low pressure cast bullet loads... The rim holds the case back where it belongs and prevent the primer from driving it forward.

only make guns in a caliber other than 7.92 x 57? Could it be an 8 x ?? to comply. That is assuming the gun was sporterized by some German gunsmith in the 1920's or late teens.

It seems to me there was an 8x54 or something like that so they could not use army ammo.

Bob

I am not terribly bothered or mystified by the primer backing out. I've seen that on a number of cartridges, all of which were loaded lightly. And, as I have not done a chamber cast, there very likely could be headspace issues.

The bit that eludes me is what headspace issue or firing condition causes the case to shrink?

If, for any reason, the case is forced into the chamber far enough to set the shoulder back, then the neck gets longer, right? That brass has to go somewhere...

The only way I know to make a case shorter, other than mechanical trimming, is to neck it up or straighten the sides. Or, in other words, take brass away from the length and put it into the diameters.

As to the query if I am certain the brass was correct to begin with, no, I did no measuring. I can only say that all the remaining 8x57's in the house are about the correct dimensions. I could try and repeat the exercise and see if the results are the same...

I will note that a few different 8mm bullet and case combinations on hand chamber without trouble so the chamber is at least as big as the 8x57. Also, the fired case definitely sits deeper in the chamber than an unfired one...tough to accurately measure but by about the same 0.100".

I will do a chamber cast if, for no other reason, than to satisfy curiosity.

I very much appreciate the thoughts and suggestions.