Originally Posted by Mike Irwin
Given that a cartridge case and primer are all brass (I believe even the anvils in most primers are non-ferrous), would that even be possible?
Electromagnetic radiations are attenuated by a "Faraday Cage". Static charges are effectively blocked, but unchanging magnetic fields (a magnetic compass still works inside a Faraday Cage) still get through and very strong magnetism gets through. If the anvil inside the primer were ferrous (or any constituent of the priming compound were magnetic), it would be affected differently than brass and possibly squeeze the priming compound to the point of detonation. But I am just speculating there.
Iraiam is probably right, but nowhere have I found that spontaneous ignition of the primer was absolutely ruled out. A firing pin indentation would prove that this discharge was accomplished that way. Lack of the indent would definitely prove a less conventional ignition.