As far as I can tell, it is a Colt Model 1855 (Root model) half-stock sporting rifle. It could be a cut down musket but the foreend looks like the sporting model. The caliber would be .56. They were made in .36 and .44 also, but those are six shots and the pictures show a cylinder that is five shot and obviously bigger than .44. As others noted, the rear of the cylinder pin is missing and the trigger guard is a replacement. The name "Root model" comes from the design which was the work of E.K. Root and Colt himself.
The patent marking should read "1856", not 1858.
Some Colt revolving rifles were used in the Civil War. They were the first rifles issued to Berdan's famous Sharpshooters, though they later got Sharps rifles (the names are coincidental; the term "sharpshooter" for a marksman was in use long before Christian Sharps was born). It is unlikely that a sporting model would have been issued, but a soldier (or more likely an officer) might well have carried a personal weapon, even if he had to cast his own bullets.
Jim
