I've done a few practice runs with my shotgun. I always place it in the mouth as it is hard to hold a long gun to one's head while stabilizing it (maybe someone has done it that way before, but afaik it's more difficult so barrel in my mouth is the way I go). Anyways, I've done it standing up and also sitting down. I try to tilt my head forward a little bit when I place the barrel in my mouth so that I will be more likely to my brainstem as well as the spine (if a near miss), as well as aiming a bit lower than higher. At any rate, 00 buckshot in a 12 gauge shotgun is extremely likely (99.9-100%) to be very fatal given the way I've set it up. If I don't die instantly, at best I will still die of extreme trauma (extensive damage to the brain itself -- virtually mush and guts) as well as blood loss. I'd imagine the moment I pull the trigger, I will lose conscious in 0.1 seconds or so.
I suppose with a pistol, yes, one could place it against their head, behind the ear as a handgun is easier to place and position than a long gun.