Press the following buttons in the correct order as shown. Do not press these buttons while your game is on pause.

Hold L&R and Press Right C
Hold R and Press Up on +Pad
Hold R and Press Down on +Pad
Hold R and Press Down on +Pad
Hold L&R and Press Right C
Hold L&R and Press Left on +Pad
Hold R and Press Down on +Pad
Hold R and Press Up on +Pad
Hold L&R and Press Right C
Hold R and Press Left on +Pad

This is not an infinite ammunition cheat.