I had my first ever ruptured primer yesterday. I was re-zeroing my No 1 MK III with some white box Winchester FMJ ammo, muzzle velocity about 2400 fps. I guess my firing pin wasn't rounded enough or the primers were a little thin. I didn't notice anything when I fired but when I opened the bolt to eject, smoke came wafting up out of the magazine. Sure enough, the primer was pierced.
Firing pin protrusion was .045 inch and I could see a tiny dimple in the tip of the firing pin carved out by the powder gas. After a few minutes work with a stone the pin was smooth and well rounded. I had no more pierced primers after that.
I guess what impressed me was how well the action vented the gas. I didn't feel a thing. I've always liked my SMLE and this just gives me another reason why.
By the way, for giggles I checked the headspace. The bolt would barely close on a .070 thick disk. I know that's on the high end but with neck sizing I've never had a head separate, even using S & B brass.
