I have often wondered why assault charges are not pressed more often, with online and TV pranksters alike. As Bullcamp points out, scaring people with what they perceive to be a weapon constitutes assault, menacing, or similar in several of the states I have called home over the years.
Assuming I did not realize such an incident were a prank, the hoaxter could find himself in situations ranging from sprawled out on the ground, with sore parts and a missing "weapon," to staring down a muzzle, to (worst case) sporting spiffy new gunshot wounds.
I might even feel sort of bad about it, afterwards.
Hopefully, nobody ever tries such a thing; hopefully, if they do, I realize it in time.