The Unlimited Submachine Gun is an unlockable firearm in Silent Hill 3. It is unlocked when Heather Mason defeats the final boss with a melee weapon. The player will then have to play Extra New Game to use it. It is found once the player exits the mall through the window and heads left into the alley. The Unlimited Submachine Gun, just as the name says, has automatic fire, and it has unlimited bullets.
Finding the Unlimited Submachine Gun in the HD Collection unlocks the trophy/achievement "Heather's Got A Gun".