Uh Darkhalo, by definition anything called a 'rifle' has a rifled barrel, regardless of how you load it.

Muzzle-loading rifles were in common use by the American Civil War, and had been for some time; the British Army had been using them on limited issue at the turn of the 18th/19th century as a skirmisher's weapon (see: the Sharpe series, which is both awesome as all hell and set in the early 19th century when military rifles became a thing). The Civil War was an absolute massacre because both sides were trying to conduct close-order line battles with weapons that easily had the accuracy and power to kill at that range; remember also that repeating rifles were beginning to appear around the same time, and were occasionally used.

If the Civil War had been fought entirely with smoothbore muskets, there would have been a hell of a lot less deaths. Keep in mind as well that developments in military equipment always predate the tactics which use (or counter) them; nobody knew how to effectively use tanks until quite late in WWI, and it wasn't until WWII that combined arms doctrine was fully established, first by the Germans in their early conquests and then mastered by the Red Army in 1944-45.