Yes, it .58 cal. and if you intend to try to shoot it it should be taken to a knowledgeable gunsmith with experience dealing with original black powder guns for a careful examination.

It may be a Colt Special Model musket as gsimmons suggested above, however, better shots of the assembled lock area are necessary to confirm - the bolster where the nipple seats should have a flat face with a national eagle stamped on it. If the bolster has rounded face and has a clean out screw, then the Colt lock has been placed on a standard M1861 Rifle Musket. It is interesting to see the "N.J." stampings on the barrel and stock, they indicate that the gun was the property of the State of New Jersey and was issued to their troops during the ACW.