You tilt the barrel up, pour in a bucket full of plain water, thumb over the touch, insert the mop and push as hard as you can. The mop is a tight fit, eventually it reaches the bottom when you start to pull.
Miraculously most of the water stays in the bore, after a few in outs you remove the thumb and push as hard as you can.
Depending on how much beer you have consumed you may, or may not, remember to warn those standing behind ::
I've never had much luck with plain water in my shotguns, even if I pump them there's always a greasy black residue. However it seems to work fine and dandy in a cannon.
Maybe I need a 16 gauge sheep :hmm: