The Turbocharge is an item that increases the speed at which the Machine Gun recovers ammo. You can obtain it after you have beaten Curly Brace in the Sand Zone, when she will offer you to trade your Polar Star for her Machine Gun. Later in the game, if you go to the Labyrinth Shop and talk to Chaba, he'll give you the Turbocharge. If you don't agree to trade your Polar Star you cannot obtain the Turbocharge. It is a little known fact that the Bubbler and Spur also speed up in reloading and charging respectively when this item is equipped.