OKay, for this story of mine called Balance. Here's the explaination I've got for it. With a forged weapon you need the intent to kill, so in fact, killing a fiend or devil is in fact a type of magic. The reason for this is from the start of the weapons life it is made with the sole intent to kill and harm. With a gun being manufactured, there is no 'will' in the weapon to do harm. Yes the weapon is extremely dangerous, but it doesn't have the killer edge of say, a sword. As the for the vampires, mages and zombies, that's because they are, or were, stock standard humans at one point or another.