That's to big a gun

[TorpX]

I see what you mean. I don't know how the would access the breach. Perhaps it was some kind of experiment?

Had a close look with the pic zoomed to 300% and not convinced that it's a gun at all.

- The conical shape of the "breech" end is unlike any gun in my fairly extensive database;

- There is no evidence of any breech mechanism or accessories that are characteristic of where typical breech mechanisms might be fitted;

- The picture and the ship it's mounted on has a late 19th or early 20th Century look although such things can be photo shopped easily enough. If it is a large calibre barrel of that vintage it lacks the telltale Armstrong or Krupp multi-tube construction(1) that characterized all large calibre guns until monobloc(2) construction and autofrettage(3) were developed well after WW1;

- If a gun, its trunnions(4) are too far forward to be in anything resembling a firing mount as demonstrated by the location of what appears to be the breech. If it's not a firing mount, why is it mounted athwart the ship and not on the centre line?; and

- There is no visual evidence of an elevating system of any type. Forward mounted trunnions would require a considerable equilibrating(5) system to compensate for the mass at the breech end.

It actually looks to me to be more of a structural component, perhaps the leg of a tripod mast. It may well have been some sort of intelligence deception but it is really hard to see it as a usable weapon or part of a real-world weapons system.

At least based on that one photo.

Edit:

I suppose there is an off chance that it might be some sort of "Dynamite Gun" or derivative. These low-pressure "guns" did not require the sophisticated construction techniques of conventional artillery but were also essentially useless as weapons of war.

http://en.wikipedia.org/wiki/Dynamite_gun

Notes:

1. Large barrels were generally built using a process originated by William Armstrong where successively larger but shorter tubes were shrunk over the breech end of the inner tube that carried the rifling. This produced barrels of great strength and also great weight. The general difference between British and German techniques is that the former wire-wrapped the inner tubes near the breach end for hoop strength while the latter used much tighter shrinking to achieve the same effect.

2. Later developments in metallurgy allowed guns to be cast in one piece; hence monobloc construction.

3. A method for providing hoop strength on the barrel to allow it to withstand the forces of firing. It performed the same function as wire-wrapping a multi-tube barrel.

4. The point around which the barrel rotates to achieve the required angle of elevation. On old cannon, trunnions appear as studs on the sides of the tube that connect it by brackets to the carriage.

5. Modern guns are generally trunnioned near the breach end, far back from the centre of gravity. This requires some sort of balancing mechanism to compensate for the unequal mass and in gunnery, these are called "equilibrators".

[TorpX]

As you say, it seems very impractical. On second thought it looks like it may be some kind of mechanism for lowering or retrieving things (boats, mines, something else?). If it was a gun, might it not be too heavy for the size of the ship? Can't really tell from the picture though.