I am guessing the propellant charge leaves no residue tha might preignite the next charge, since there was no quenching mechanisim  visable.

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Note they are not swabing the breech with water.  Which makes for a slight risk of misigniting the charge.  Perhaps these newfangled charge bags dont leave sparks?  The Marine firing the cannon is not using laynard, letting his hand ride the firing lock.  He's getting away with it with the low charge but I'd not want to try it with a charge 7.