The Pineapple Launcher is the name of Chunky Kong's personal weapon in Donkey Kong 64.
The Pineapple Launcher is a single hand bazooka that shoots pineapples. How Chunky fires his Pineapple Shooter is actually unknown (because there doesn't seem to be a switch).
The Pineapple Launcher can be purchased by Chunky Kong from Funky Kong for three coins at Funky's Armory in the area Frantic Factory or any other area . With the Pineapple Launcher Chunky gains the ability to shoot Banana Balloons, hit Pineapple Switches and snipe enemies from afar.
Further into Donkey Kong 64 special upgrades to the Pineapple Launcher (as well as other weapons) can be purchased at Funky's Armory. Some of these upgrades include a larger ammo capacity, a sniper scope and enemy-seeking ammo.
Trivia
This weapon makes an appearance as an item in Donkey Kong Barrel Blast. Ironically, Chunky Kong does not appear in this game. It could've been possible the Pineapple Launcher would've looked like a real bazooka in the beta version and changed it for the E ratings. However, it is not clearly confirmed.