Hold RIGHT TRIGGER then quickly press X, B, B, B, X, B, Y, A.
Nothing will confirim correct code entry.
Enter this code during the game while holding right on the D-pad.
B, B, X, Y, Y, X, B, A
When the game's Legal Screen (copyright information) appears press and hold down A, B, X, Y
right d-pad; XX YY BBB A
repeat to upgrade level 2 weapon
repeat again to upgrade leavel 3 weapon