In the mouth is probably the best way to do it, because you’ll be directing all that force in the direction of the base of the brain and the brainstem. What we see fail is when people put it up under their chin. Don’t get me wrong, it’s still catastrophic, and you’ll probably die eventually, if you don’t completely occlude your airway right away. But it’s gonna be an agonizing wait.

I’d imagine sidearm failures are due to the curvature of the skull. It’s not unheard of for a round-nose projectile to skip across the top of the skull (figure it’s a super-hard rounded surface, and a bullet is going to prefer the path of least resistance) rather than penetrating, especially if you’re doing the classic barrel-to-the-temple move, and you limp-wrist it at the pull, pointing the barrel upwards.