Tragic,a most preventable event.Yes,I agree that many things were done wrong,even with a proper cannon.The above link should be in the appropriate sticky IMHO.
The cannon was not designed for live fire,it did not have a proper seamless steel liner.
The bore looks to be 1.25-1.5" if they were stuffing 35mm film canisters in it.FFFG in this bore range is unacceptable,900grains of FFFG :shock: .They were using volume measurements and not weight,you should weigh your charges in the large bores.Small cannon bores that equal muzzle loading rifles/pistols of equal or better strength volume is fine with appropriate charge.
Stuffing toilet paper and plastic bags or using any wadding is dangerous and should not be done.
The report mentions the bore may have been improperly maintained.Use a worm to remove foil powder bag pieces after firing,use a wet/dry mop proceedure to clean between shots.Clean bore thoroughly of powder residue and use a rust preventative.
The report also mentions the need to make sure your powder charge is seated correctly and no airspace exists if using a projo.This is BASIC muzzle loader safety whether it is a rifle,pistol,shotgun,cannon,mortar or hand-gonne.
I really hate to see things like this,as they are preventable with proper education and training.