There were some modern style weapons used during the CW besides the Gatling Gun:
The Henry repeating rifle, precursor to the Winchester 30-30, was used by Union troops by 1865. This rifle used a modern style metal casing for the bullet and primer.
The Sharp's Carbine, used mainly by Cavalry, was one of the first mass-produced breech loading rifles.
Though most soldiers still had muzzle loading Enfields and Springfields, they were expected to get 3 shots off per minute. This can be done, for I have demonstrated this (using blank ammunition) during CW events.
