I'm not sure I buy it.
I volunteered to be tasered during the training module while I was serving as a police reserve. They hooked alligator clips to my shoe lace and belt loop along one leg and gave me a full 5 second ride. It's the most excruciating pain I've ever felt, but there was no heat to the process. Watts instead of volts is what makes your muscles lock up. Maybe someone with more knowledge in the field of electricity can shed some light on how it works, but the way I understand it, the charge would have to travel through the primer directly, taking the path of least resistance, which it does unerringly, and it doesn't make sense in the realm of physics to have a whole mag cook off, that being the case.
For sure don't take my word for it though, I'm just a moron with an internet connection.