Barrel Cannons have been in all Super Smash Bros. games to date. Using one causes you to get inside the Barrel Cannon, and it will spin around. If you press A when inside, you get shot in the direction the arrow on the barrel is pointing. If ypou don't press A for a while, the barrel automatically fires you in the direction it's pointing. It turns you into a projectile for a little while, damging foes. Also, you'll have to tech to recover.
These are permanently placed on the bottom of Congo (Kongo) Jungle (N64) and Kongo Jungle (GCN) for recovery.