It appears to be a two banded, short, rifled musket, (by the sights), simular to the 1842 Springsfield short (cavalry) rifled musket. But the front barrel band is not correct for that weapon. I don't believe that it would be a weapon that came from the Little Rock Arsenal. I don't believe that any such weapon was in their inventory but maybe Anthony Rushing could correct me on that.
One thing about these photographers they usually provided their own weapons for these types of Photographs as usually weapons were left in camps. We don't know where is photograph was taken or when. As you have already noted about the Bayonet the weapon is probably European in origin.