You have to be in the engine room before fighting the B.O.W. on the 12th save point. Die on purpose when fighting the B.O.W. Use a continue. Get the handgun bullets and keep doing this until you have 99 handgun bullets.
Unlimited Gas Launcher rounds
First, equip the Gas Launcher with at least one gas round. Then, engage and enemy in battle, and use the 'run' feature with the character equipped with the Gas Launcher. When in 'run mode', pushing the action button will cause you to fire a gas round without depleting your ammo count, thus killing all on-screen enemies.