Most boys learn this early in life! 
Muskets must shoot knuckle balls. The knuckle ball is unpredictable just because it goes whichever way a slight air current sends it. The spiraling on a rifle keeps it from knuckling. And the spin on a curve ball keeps it path fairly predictable. Interestingly, if a curling stone is thrown without any spin it will also follow a pretty random path owing (presumably, since air currents couldn’t possibly affect it) to minor imperfections in the ice. A heavy spin will–surprisingly–cause it to go nearly straight.
My dad put together a 30-'06 out of surplus parts right back in the early '60s about the time I was born. When I was in my early teens, he tried to explain to me how this worked. He told me that this rifle fired a bullet with a trajectory that, with the scope settings, crossed zero at 20 yards and again at 200 yards. At 100 yards, it was six inches above zero. He said that if I ever needed to sight it in, just do it at 20 yards because it was easier.
I didn’t much understand it at the time.
Fast forward 20 something years - After a couple of physics classes in college and after my dad had passed on and I inherited the rifle, I took it out to the range one day, and I’ll be darned if he wasn’t exactly right. At 100 yards, it was exactly six inches high. I had considered working up a different cartridge load, but I figured why mess with success. Later, I was going through some of his reloading notes and I saw how much trial and error he had put into developing with that load.
It is not so much air currents as slight imperfections in the round projectile. Rifles designed to shoot round balls have very modest twist. The goal isn’t to stabilize the ball as required with a cylindrical bullet, but to average these errors so they don’t build up…the ball flies in a slight corkscrew path rather than a pronounced curve.
As for the ceteris paribus comments: Shooters appreciate how much easier it would be to get a bullet to the center of a target if the real world would just work like a problem in physics class. It is fine to ignore aerodynamics when explaining gravity, but less so when explaining ballistics.
Finally, to really screw things up:The sight line may often not be straight. On real world rifle ranges the optical sight line often curves due to temperature gradients in the air near the ground, AKA mirage. For this reason, a light steady wind (which breaks up the gradients) may allow more accurate shooting than dead calm conditions.
No. Muskets fired round balls that were intentionally smaller in diameter than the barrel. The black powder they used at the time quickly fouled the barrel, and a tight fitting round gets almost impossible to load after you’ve been shooting for a bit. This was why they used smooth bore muskets instead of rifles long after rifling had been invented. Rifles on the battlefield were almost worthless after a few shots because a rifle round has to fit tight or it won’t grip the rifling and spin (which is the whole point of rifling), and after a few shots you can’t load a tight fitting round.
The looser fitting round ball would randomly contact one side or another of the barrel as it made its way out, imparting a random spin onto the round. Unlike a rifle round, the random spin from a smooth bore musket isn’t around the line of travel, so it makes a curve ball. The round will typically go straight for maybe 50 to 75 yards, and then curve off in some random direction. They used to say you could stand 200 yards from a single musketeer and not fear getting shot by him.
The invention of the Minie ball in the 1840s changed muskets rather dramatically. The Minie ball wasn’t a ball at all, but instead was a conical bullet with a hollow skirt. It was intentionally a smaller diameter than the barrel so that it could be loaded even if the barrel was fouled by powder. When fired, the skirt would expand and grip the rifling, giving the round a spin along the axis of travel, which greatly improves its accuracy. Instead of curving off one way or another, the round spirals through the air like a football, keeping it on its path. Even a badly unbalanced round will fly in a corkscrew pattern through the air, ending up pretty close to its target.