This bullet Bill cannon moves up and down along a wall, and stops when it hits a floor or ceiling. Its X position determines weather it shoots right or left, but the extra bit sets it to shoot towards Mario always (and changes the palette). Speed, firing rate, and palettes can be changed easily in the CFG or ASM file (depending whether the cannon shoots only in one direction or towards Mario).
This resource is identical to the Wall Scaling Bullet Bill Shooter by the same author. The original submission was accidentally updated years ago with the wrong files, and it's now unlikely that the original code can be found again.