Clyde,
From what I can see in the pictures that you've posted, you have a .577 caliber, Model 1855 Enfield rifle, imported from England. It is not a truly rare rifle, but in fact, was the second most widely used rifle in the war. But, unfortunately, I can also see that you are missing a few parts for yours. You may want to consider having an expert look at it to see about getting the original parts to restore it. The 25th Michigan Infantry were issued these rifles in 1862, the same year the regiment was mustered , so it appears to be an original issue weapon. But I defer to those more knowledgeable out there who may also shed some more light on this gun. Hope this helped.....George Wilkinson, collector and historian of the Fourth Michigan Infantry