Nat digs out a bullet, holds it up and says "ah ha, M-16". The M-16 is a type of rifle that fires 5.56mm bullets, NATO rounds. Dozens of different guns use that same ammo. While a good eye might tell that a bullet is 5.56mm it could not tell what gun fired it.
The existence of empirical evidence apparently makes it hard for a vampire to make someone submit their will and forget that vampires are real.
The flashback scenes take place in Virginia, USA in the early 1860s.
There are vampire police, of a sort, called Enforcers who kill humans who find out that vampires exist.