It is a .58 caliber Springfield rifle. This one was made in 1861, near the start of the war. It is the main rifle issued to Union troops although the Confederates picked them up by the thousands off battlefields. Springfield made about 265,000 so they are fairly affordable. In order to keep up with demand about 20 other arms manufacturers also made “Springfield pattern rifles” that were nearly identical to the original.
In total over 800,000 Springfield type rifles were made during the war.