The
tailgunner is also the person in the tail of a combat aricraft, usually a
heavy bomber such as the
b-17 or the more modern
b-52 who augments defensive capability for the
aircraft, namely preventing the
plane from being attacked from the rear. This isn't much use unless the attacking plane is coming from the rear to use his guns. This is required because the very heavy bombers cannot out-manouver the smaller and much more agile fighter aircraft.
This is also a euphemism for a male homosexual, think for a second about the stereotype and I'm sure that you'll see how it could be applied. ( this excersise is left for the reader)