Here is what happened.....

by JD, Western Washington, Thursday, November 13, 2014, 14:26 (3668 days ago) @ Hoot

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.