The Coconut Shooter resembles a hollowed out log with a with twig sticking out of it, this is possibly the weapon's crosshair. As the name would imply, the Coconut Shooter is capable of shooting coconuts.
The Coconut Shooter can be purchased by Donkey Kong from Funky Kong for three Banana Coins at Funky's Armory in the area Jungle Japes, the first area of Donkey Kong 64. With the Coconut Shooter, Donkey gains the ability to shoot Banana Balloons, hit Coconut Switches and snipe enemies from afar.
Further into Donkey Kong 64 special upgrades to the Coconut Shooter (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.
Interestingly, beta-screenshots of Donkey Kong 64 showed that the Coconut Shooter was originally going to be a weapon similar to a double-barrel shotgun.