Lead is soft. And the cannon and musket balls, before the advent of rifled barrels, had a diameter narrower than the barrel bore (windage), so the friction is reduced when extracting them. Also the pulling action may be done in a rotating mode, which also helps a lot the ball to come out without much fight and without risking to break the rod which, in any case, must be made of the hardest woods ... ebony for one.
Eventually in the cannon case and even if iron balls are used, i realize that what you extract with the worm is the wad (plug); the ball rolls out by itself, once you incline the gun.
It looks as i know the slightest thing about this issue
