I went on a day trip to Richmond with my dad to visit the battle sites of the Seven Days Battles and we hired a private tour guide to take us around the battlefields and take us to the place where a family member fighting for the Confederates died.
Among other things he mentioned that at some point in the war both sides began to equip rifles, as opposed to smoothbore muskets. Unless I'm mistaken rifles were limited to sharpshooters early in the war.
From what I've read rifles are much more accurate than smoothbore guns and have greater range so, assuming that what the tour guide said was true, what did the widespread introduction of rifles to the Civil War do to casualty rates and tactics in general?
Rifles were standard issue during the Civil War, with the 1853 Enfield and the 1861 Springfield being the most common issued. You are correct that rifles are more accurate and have significantly greater range, however, this was rarely factored into the tactics of the day. It was common for an attacking force to begin the assault with one round loaded, get into firing range, and then charge. The intent was to close the distance as rapidly as possible to limit the advantage of defensive artillery, however until the gap was closed the defenders had the advantage of continuing to fire upon the charging force leading to consistently higher casualties among attacking forces throughout the war.
It was not until after the Civil War that there was significant adjustment of infantry tactics away from the Napoleonic Warfare. Emory Upton led a group that published a new manual for infantry tactics in 1867, entitled A New System of Infantry Tactics, Double and Single Rank, Adapted to American Topography and Improved Fire-Arms which intended to take advantage of the range and accuracy of the rifle. The Army incorporated these as standard doctrine fairly quickly. Upton also recommended a series of other changes to the way the Army operated, including fixing personnel management and supply but these were ignored, which contributed to Upton committing suicide in 1881. The Army would ultimately encompass many of those changes in the lead up to the Spanish American War.