I believe that the idea of rifled barrels was known earlier than the American Revolution, but they were by no means universally used.

Rifled barrels were used for hunting weapons, especially small-game ones (the Kentucky "squirrel rifle") because of the greater accuracy, needed for hitting a small target at long range. The squirrels and rabbits would bug out on you if you tried to get close.

Unrifled muskets were still the common military weapon at that time, for three reasons:

1) Infantry tactics at the time involved troops in ranks, firing in volleys. Accuracy of each individual weapon was not that important.

2) An unrifled muzzle-loader could be reloaded faster, since you didn't have to force the ball down the tube against the resistance of the rifling.

3) They were cheaper and easier to manufacture.

One of the main reasons the Americans won the revolution was that they didn't know diddly about infantry tactics, so they did the rational thing: hid behind something that would provide cover from enemy fire and concealment from observation, and fired as individuals, when they'd get good shot, at ranges the unrifled Brown Besses couldn't reach with any accuracy.

Or so I've read.

Peter.