Armstrong 100-ton gun being lowered on board of the Italian transport ship Europa, United Kingdom, probably 1870s.

This was a 17.72" muzzle loader as seen on the Duilio & Dandolo Italian predreadnought battleships, the so-called 'monster gun' that defined a short-lived era of battleship construction.
Gun details: http://en.wikipedia.org/wiki/100-ton_gun