There were numerous trials by the War Dept. of breechloaders of different kinds, Colt's cap-and-ball style among them, and in 1855 some Colts were handed to troops. There were two main problems noted with them; 1) they were rather complex, so that soldiers trying to strip them down, who were used to simple musket locks, had problems re-assembling them and 2) they could chain fire- a discharge of one chamber could spread to the rest. This could damage the gun, damage the soldier, and at the very least the recoil was very unpleasant ( like setting off both barrels of a double-barreled shotgun today).

Later, when metallic cartridges came in, there were some revolvers that were long ( Colt had the famous Ned Buntline model) but a magazine rifle like the Winchester could hold as many shots, handle more powerful cartridges, and there was no leaking of gas between the cylinder and barrel.

Claude Fuller: The Breechloader in the Service