Probably anticipated an altercation with a black guy that had boss-level HP and he was playing on Easy so he had unlimited ammo. Also he might have input the code to play as Goro.
He's just exercising his God-given second amendment rights, I'm sure he has a very logical and plausible reason for carrying that much firepower, like growing his weaponry collection since he's grown too attached to guns or he's simply hunting down Pennywise, that fucker won't let him sleep.
I was just talking to my girlfriend about this - I am of the opinion that anyone that feels the need to carry around or possess that much ammunition (without expressed purpose, obviously) has to have some sort of instability. They may ultimately be harmless, but I can see no other reasoning.