A skull barrel is a hazard that has appeared only in Cactus McCoy 2 so far. Skull barrels are especially found in the elevator section of Blackstone Tunnel, the sixth area. Since it kills McCoy upon contact, the skull barrel is the most dangerous hazard in the game.
In terms of shape, a skull barrel is very similar to a TNT barrel. However, it is painted red and it has a white skull drawn on it, signifying that it causes immediate death.