Barrel Cannons debuted in Donkey Kong Country as a means of travel across large gaps and as a means of accessing bonus games. Present both singly and in "chains", jumping into one would leave a character sitting inside the barrel until they were fired. Many variations on the basic theme existed, including fixed cannons, autofiring cannons, cannons which rotated, cannons which moved back and forth in a line, and combinations thereof. Donkey Kong Country 2: Diddy's Kong Quest introduced arrow barrels (image to the left) that indicated which direction they would fire, which became the basis for the Barrel Cannons in the Smash Bros. series.
While its exact behavior varies slightly between games, it operates the same overall. Characters that enter the barrel are temporarily held inside. Pressing any button or waiting long enough will fire the character in whatever direction the barrel is pointing, sending them reeling; depending on the game, the character might deal damage to enemies they run into.
As an item
In Super Smash Bros. Melee, Barrel Cannons appear as a rare and quickly-vanishing item, appearing infrequently even if the item drop frequency is set on very high. They can be picked up and thrown as heavy items, or they may become active should they appear on a sufficiently steep slope. Characters other than the thrower that touch an active Barrel Cannon will be trapped inside, and must press a button to fire themselves to escape. Waiting too long to fire may cause the barrel to roll off the stage, making it very difficult to recover if it subsequently fires in the wrong direction. Touching an occupied barrel deals minor damage and knockback. Giga Bowser, Master Hand, and Crazy Hand cannot enter a Barrel Cannon, so they will take damage if one is thrown at them instead.
The bottom of the cannon has the text "2L84ME" on it, which translates to "Too late for me", referencing the difficulty dodging and escaping the Barrel. The same alphanumeric sequence appears on a wooden crate in a piece of official art for Donkey Kong Country.
As a stage element
Barrel Cannons appear in various Donkey Kong-related stages.
In the two Kongo Jungle stages, a single Barrel Cannon floats below the stage, moving back and forth. Characters can enter the barrel at any time to be fired out a short period of time later, or after they press a button. While the Barrel Cannon can be used to recover if the character can't reach the stage, it can rotate unpredictably, resulting in being fired sideways or downwards to a sure self-destruct. Characters cannot enter occupied barrels.
In the case of Kongo Falls, a Klaptrap brought down by the rapids can occasionally become stuck in the Barrel Cannon. Touching the Klaptrap-occupied barrel will deal damage to the character in addition to denying entry. Eventually, the Klaptrap will be fired, dealing 30% damage and incredible knockback if it happens to hit anyone.
Barrel Cannons in the Jungle Hijinxs stage have the "starburst" design from Donkey Kong Country Returns. They are used to travel between the stage's two planes. Characters that attempt to use a Barrel Cannon too soon after the previous one (due to still being smoking from the first) may cause the barrel to explode and launch them upwards or downwards, stopping players from simply using them to constantly run away from opponents.
The Subspace Emissary
Barrel Cannons appear in The Subspace Emissary as black, metallic tubes with yellow arrows. They operate in exactly the same way as in the Donkey Kong Country series - certain parts of certain stages involve precise timing to traverse a chain of Barrel Cannons. Barrel Cannons with a blue highlight under the arrow may be fired manually with no maximum wait time, while others fire immediately.
Certain Barrel Cannons act as traps, firing the character offscreen during ambushes and the like.
Barrel Cannons appear in Smash Run, using the Subspace Emissary role and design. Unlike Brawl, however, players cannot indefinitely stay in certain Barrel Cannons; rather, they will be shot out after enough time has passed.
Cannons similar in purpose to Barrel Cannons appear as objects in Super Smash Bros. for Wii U's Stage Builder with a unique design. They can be placed at any rotation and remain fixed, automatically firing anyone who comes into contact with them. The large cannon sends fighters flying farther than the small one.
Classic Mode (SSBU)
The Barrel Cannons were used in Donkey Kong games as a vital means of transportation, and their explosive nature is used similarly in Super Smash Bros. Melee. On the Kongo Jungle stage, one can catch characters as they fall from the main platform. Careful! The Barrel Cannon rotation may accidentally send you flying off the screen.