"April's Gatling Gun
." Maybe the villain is actually named John Harrison and the name of the gun is simply his former commanding officer's who was killed in action. Maybe Harrison thought Starfleet abandoned them, left them to die at the hands of the Klingons or something, and now seeks revenge for it?
Now that commanding officer could still have been Robert April, and maybe he was in-command of a ship named Enterprise, just not the current one, I mean is there any reason why there couldn't have been a ship with that name prior to the NCC-1701? And if so, is it still around?