Print this More Ammo for shotgun
First unlock the cheat selection by enter:
Down, X, up, Y, down, down, up, up
Then use this code to get a shotgun:
KKsh0tgun (its a zero not an O)
Then use the cheat for 999 ammunition:
KK 999 mun
Then reload you're shotgun.
Do this repeatedly and you'll have lots of ammo!