You have a rifle musket that has been converted post military service to a shotgun for the civilian market. If the barrel is still it's original length (39 inches long) it could have a forearm added. You will also need the three barrel bands, band springs, nose cap and ramrod but it will be very cost prohibitive.

Also, keep in mind that the lock (made at Springfield) came from a M1864 rifle musket while the hammer and barrel are both M1861 items. This model mixing probably took place at the time of "sporterization".
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