Wow that left thumb looks gnarly, that's guna throb like hell if it's below heart level. Been loading since the late 80's, watch my dad use one of the Lee Classic reloaders for 357 in the 70's at the kitchen table where everything is done with a hammer, heard a few pops. I personally have never had on go off. When I used a single stage I would prime with a hand primer. When my Dillon 550 primer slide gets really dirty it will get hurky jerkey and sometime a primer will flip upside down or sideways, thinking it was just a tight pocket seated it that way, yes even sideways/upside down, NEVER had one go off. Even using a sizer die to remove them, the upside down ones I reused. Priming is such a "by feel" thing, got to be careful. I feel it has to be static...based on my mistakes yet never having had on ego off.