I just realized this question makes no sense whatever. :hmmm:
I am thinking that the term '12 pounder" is what caught your attention. The smoothbore bronze artillery piece that fired 12 pounder (2.75 inch) ammunition was named in honor of Napoleon III. The howitzer in the picture, the "chunky" one, also fired 12 pounder ammunition. 12 pounder ammunition is a standard size and like most artillery ammunition was not unique to a specific type of gun.
The breechloading Whitworth is an exception in that it did not fire a standardized round. It fired an octagonal bolt that fit into the grooves in the barrel. The Whitworths came in different sized ammunition ranging from the 12 pounder field artillery piece of about 3 inches (about 74mm) to heavy artillery of eight inches. Like their rifle, Whitworths were highly accurate and long ranged.