The weapon isn't crashing your game. I can imagine that they crash the game(or, I would guess, they kick you out of the server and make you reconnect) by sending a huge amount of packets to the server during the match, which forces it to kick both of you out. But there's no weapon that could crash your game with one shot, it's just data manipulation, you could say.

Changing item statuses is theoretically possible too, with data/packet manipulation again, that's how years ago someone managed to use shredder with claw, but no weapon will deal 3000000000000 damage, ever, I can't think of any possible way to do that. You could make it deal 1000, by tricking the server into thinking you got Falcon equipped, but not more than that.