I just had another thought. If you remember from high school physics class, a rapidly compressed column of air gets hot. I wonder if maybe the act of hammering on that bullet created enough of that phenomenon to get hot enough ignite the powder? Just wondering.
If the bullet was rapidly driven farther into the case of a already compressed load the pressure in the case would have risen exponentially possibly causing the primer or the powder or both to detonate.
Member when we were kids and used to hit the the whole roll of pop gun caps with a hammer?
Ker-Pow.
