ORIGINAL: usmc1978
I don't see how it could happen, either. It's not enough to strike a .22 on the rim, there has to be something to strike against, i.e. it's a hammer-anvil effect. The firing pin hits one side of the rim while the other side is supported against the edge of the chamber. The firing pin crushes the priming compound between the brass lips of the primer.
The batteries? There's an electrically primed muzzleloader on the market, but that's black powder with an ignition system designed to set it all off.
Just because we don't understand the conditions under which it happens does not mean that it is impossible. There are enough stories around about such occurrences that we know it DOES happen.