You're right BP, they do have inbuilt safety devices just to prevent that sort of accident. Incidentally, there is also a drill for removing a live if it fails to exit the tube.
While the bomb might seem to rattle down the tube like a stone, it is fairly well cushioned by the air it compresses in the tube on its way down. On the way UP, there's a hard light green coloured 'driving' band that is opened and spreads very slightly by the explosive force to ensure that there is minimal space between the bomb and the tube that might allow excess propellant gas to escape. That's why bore measurements are quite critical and logged in the barrel history sheet/log book at regular intervals.
As BAR will confirm. There's an almost pleasant BOOOOOOMPH when you're standing close to the gun pit. But when you're close to the muzzle, as the loader is, it's ear shattering. That's why the loader drops the round in, he rolls around,drops head below the muzzle and hands over ears - as if by force of habit.
Mortars cause more casualties than any of the other Infantry weapons. Devastating until you can pin point where the gun pit is