Nice, but very plain and quite utilitarian. Except for the rear tang sight, very militaristic.
With that aperture sight being mounted in the wood and not attached to a longer tang, tells me the gun was given that sight by a sportsman after it was made.
Calibre roughly .63".
Quite likely, it has a Tige or Delvinge-type chamber.
Tige is a flat topped spike running up the centre of the bore mounted in the middle of the breech-plug, the length determined by the amount of powder used in the issue paper ctg. Likely a bullet & solid at this date.
Delvinge chamber has a powder recess in a smaller "than the bore" breech. This powder chamber accepts the powder charge. Slamming (throwing)the loading rod 3 times on the undersize bullet upsets the bullet into the rifling, in both designs
rather than having the progressive depth rifling and use of a hollow based slug, like other Euro and American countries used.
Austrian military guns used these two interesting "principals".
This use of these two systems is possible, but not positive. The gun may have been designed for hollow based "Minie-type" "balls" as well.