I have never seen this with a primer in a black powder cartridge.
Rifle is a late 1870s Mauserbuchsen, a scaled down 1871 Gewehr action.
Cartridge is 9.5x47R, an early German adaptation of the 11.15x60R cartridge.
Load was 50grs OE 1.5fg under a 250 grs paper patched slick.
Primer is CCI large pistol.
25 rounds in the same batch, only this one had back flow into the firing pin aperture of the bolt.
As per above, I have never seen what I would normally associate with an overpressure event in a black powder cartridge. And the fact that it was only one out 25 loaded at the same time suggests a soft primer rather than a pressure issue per se. Can’t get much more than 55 grs into the case even under compression, so there’s no double charge at issue.
Your thoughts?