This cheat code will allow you to fill up all of your ammo in your weapons:

Press Start to Pause the Game and then press, Y button + Y button + X button + push down on control pad + press the right trigger + press the left trigger + press the right trigger + push up on the control pad.

You will hear a beep noise after entering it. Then just unpause the game and all of your ammo for your weapons will be full.

*Note* Do not hold down on each command, just press and release each one.