(Video Link)
The folks at Springfield Arsenal in Lorton, Virginia acquired a 16th Century cannon that would be fitted onto the end of a pole. It has three barrels. Presumably the musket nipple on the side of each barrel would be slapped against a hard surface, causing the gun to fire. At 2:55, they fire it with 75 grains of black power and a .69-caliber musket ball in each barrel.
via Make
I thought the same thing about the percussion caps.
Bill