we tend to forget the details over the centuries. back then stone cannon balls were still fashionable - literally. stone was cheap & plentiful as was labour. it was forgiving as it didn't jam in the bore (it'd break before the gun would). it was lighter and took less precious powder to propel. OK against troops, walls required heavier and non-shattering ammo tho for quick results.
in those days the powder was in general pretty poor, the formulae varied to the gunners whim as well as the purity and availability of the components, which were usually mixed dry & not corned in a powder mill. dry mix tends to have the components settle with transport vibration, and the mix would vary throughout the barrel. all in all giving a very variable result.
windage was not as much a problem as we are led to believe, having fairly quickly figured out that too tight a bore fit caused explosions killing the very expensively trained gunners (some of whom were not worth the powder it took to blow them up.
note origin of this expression). too loose a bore produced a whooshy bang but little result down the field. a bright spark figured out that sticking local green grass wadding down the bore on top of the powder before the ball helped seal the bore. wanting perfection he also would top the grass with a layer of soft malleable river clay. sadly as they had to wait for it to dry, it seriously impeded the rate of fire, especially on larger siege gonnes. they eventually scrapped the clay.
naval cannon would of course add a layer of wadding over the projectile to keep it or them from rolling out of a depressed muzzle, which was very embarassing for the gun captain when it happened. if it happened to land artillery with an advancing enemy, the targets would frequently show him the error of his ways. survival of the fittest again resulted in leaps in technology.
thus finally later entered the pre-made round. silk bags of pre-measured and graded corned powder allowed rapid reloading and charge adjustment. the final load would be a silk bag of powder attached to a sabotted ball, the sabot made of wood, attached to the ball by two tinned sheet iron straps. the ball was inside the silk, so could not dribble out a downward leaning muzzle. the sabot/silk bags sealed the bore.
result!.
just as they finally figured that out someone went and re-invented the breech loader with percussion primed brass cartridge cases and ogival streamlined exploding projectiles with copper sealing bands to engage the new-fangled rifling, and the art of gunnery changed forever. for one thing, the projectile can now never dribble out the front end of the gun.
and then we forgot.
i watch the american tv series 'mythbusters' on satellite tv here and shudder when they make a cannon, either compressed air or chemical explosive driven. they are forever just ramming a ball (golf,pool or tennis) down the muzzle, ignoring the windage and wondering why they never get a good result (tennis balls work best as they, being hollow, can deform with pressure more easily to seal better with their fuzzy outer jacket).
rather than adding a sabot or wadding or both they usually* decide that the fix is to add more power - more charge, higher air pressure, or more modern propellants, such as dynamite (which alfred nobel early on found exploded too quickly to be a good propellant, exploding the gun before the projectile could move - the dynamite gun was an abysmal failure). bigger is not always better.
*-the cute ginger headed girl one has been known to use a sabot so i know she knows what they are. they don't remember often enough tho.
p.s. - i like things that go boom. sadly now restricted here in the UK to cheap imported chinese fireworks of minimal bangs during holiday seasons. back home in alabama i got to shoot and blow up stuff much more effectively. i recall with fondness our first howitzer made from 3"-ish pipe that just took a pool ball. we'd drop an illicitly obtained cherry bomb or ash-can firework with lit fuse down the bore, quickly followed by the pool ball (no wadding - we had to be quick). worked a treat. me and the participating culprits lived on a small lake, the balls easily made the other side a couple of hundred yards away. we bombarded the construction site building the new and hated high school on them other side. having one brain cell between us, we used it to make a good descision to only do it after the workers left for the day. the brain cell was insufficient to figure out that when we ran out of pool balls, and our supplier's dad couldn't find them when he wanted to play pool that weekend, we'd be in deep doo-doo. no more cannon. we made rockets instead. and that is another story best left for later.