If you watch a round ball fired from a smoothbore, using a pair of biocs or spotting scope for viewing, sometimes you can see it take on a curve like a turmpet's horn, verring off to one side, heading down into the ground, or up at an angle before it disappears from sight or into the ground - at 80 yards or further out is the best place to see this. Other times it just seems to go in a straight line off target, even though the person shooting said it was a good hold. Sometimes it curves onto the target from a line originally heading off.
they are really quite fun to watch sometimes. Much depends on the quality of the casting, sprue, if any and of course, if loose enough to bounce against or off a barrel wall on it's way out (just wads, no patch). I tried just that in my .44 - no patch, just over powder and over ball wads, horrid accuracy resulted even at 25 yards, yet the ball was only .007" smaller than the bore which is quite tight as smoothbore balls usually run, actually. with patches, it would shoot 1" groups easily at 25 yards off bags. This is why I say you cannot tell anything about accuracy loads with a rifle at 25 yards - even smoothbores can shoot 1" groups. Ragged holes only 1" in diameter, with a 20 bore yet.
Much also seems to depend on the speed. The faster the ball is going, the further out they canget before 'problems' with atmosphere or ball send them heading off for who knows where. This is why the US Military thought their .69 caliber muskets should be delivering 1,700fps - to give longer accuracy range - hense the 165gr. charge in the paper ctgs. for charge and priming. Later, when powder quality improved, that was dropped to 135gr. - still a mite higher than most guys shoot in their guns today - I'll warrant. 165 and even 135gr. will kick in a gun not designed for it. Their military guns were not. The 6 1/2 pound carbine of 1847 might have been the worse - perhaps. Many of them had both front and rear sights, even though left as smoothbores after the 1850's swtich to rifling the current muskets for the minnie.
That would make them smooth rifles, wouldn't it.