From the Super Mario Wiki
The Pineapple Launcher is a single hand wooden bazooka that shoots pineapples. How Chunky Kong fires his Pineapple Launcher is unknown because there does not seem to be a trigger.
The Pineapple Launcher can be purchased by Chunky Kong from Funky Kong for three coins at Funky's Store in the area Frantic Factory or any other area after Chunky Kong has been rescued. With the Pineapple Launcher Chunky Kong gains the ability to shoot Banana Balloons, hit Pineapple Switches, and snipe enemies from afar. The Pineapple Launcher is the most powerful weapon used by the Kongs in the game's Adventure Mode. In the multiplayer mode, the Orange grenade launcher used by Krusha is the strongest weapon.
Further into Donkey Kong 64 special upgrades to the Pineapple Launcher (as well as other weapons) can be purchased at Funky's Store. Some of these upgrades include a larger ammo capacity, a sniper scope and enemy-seeking ammo.
This weapon makes an appearance as an item that appears from a blue balloon in Donkey Kong Barrel Blast. The Pineapple Launcher fires a pineapple at the racer in front of the player, though opponents can jump over the pineapples. When used by a Kremling, the Pineapple Launcher fires two pineapples at once. A player that gets hit by a pineapple will lose ten bananas. If a pineapple cannot find an opponent, it will home into the nearest barrel and destroy it.
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