I only wish my maths was up to calculating the force needed to raise a 60" shell to whatever the launch velocity needed is in the length of a mortar tube. and calculating the rise height available with gravitational and air friction deceleration. On one trip to Royal Gunpowder Mills (Waltham Abbey) there was some replica BP grains from WW1 large naval guns. Those grains were 14mm AF hexagons about 12mm thick. This was to reduce the muzzle thickness by distributing the thrust as the shell (iron) moved along the barrel.