[HeadUp]

that is true about the severed head functioning for roughly 15 up to 30 secs
and there are plenty of receptors in the neck, so no doubt, for the insant that it happens it hurts like nothing else, although, youre right you die soon after so it makes no difference

but not all gun shot head wounds can be fatal
gage's wound went through the left cheek and out the top of his head, the rod was over a footlong, propelled by a dynamite explosion

so depending on the location of the shot, it is possible to survive
i think that the only lasting effects would be for the brain and its functionality

its wierd tho that it might be fairly painless