I believe you might be thinking about either the 15" Rodman Gun of which there are 6 there at Fort Jefferson, Dry Turtugas or the 10" (300 Pdr.) Parrott Seacoast Rifle of which there are 4 at the fort. The only other ordnance there is one 24 Pdr. Coehorn Mortar and a 24 Pdr. Siege and Garrison Howitzer M1844. In our travels around the country, Mike and I took pictures of all, but we will post the two biggest as these would be the most memorable. We still have not been to Fort Jefferson, Dry Tortugas, but RocklockI is determined to sail his yacht named "We Go!" to the Dry Tortugas fort from the west coast of Florida. Mike and I are the crew. Oh goodie..........
The huge 10" Parrott on a display mount in a small cemetery in Trumansburg, NY in the Finger Lake country. Weight 27,000 pounds.
There are four of these at the fort, all unmounted. With nothing around it for scale, Mike and I placed ourselves there.
Our image of the first 15" Rodman Gun produced and now at Fort Monroe, VA is temp unavailable so I am using one from the Marker Hunter website here:
http://markerhunter.wordpress.com This so-called, "Lincoln Gun" was the only 15" Rodman ever used in the Civil War. It fired from the "Beach Battery" at Fort Monroe, over Fort Wool on the rip-raps, all the way to the Confederate batteries at Sewell's Point on the south side of Hampton Roads. The results were negligible.

We are including this one I took in 2008 at Fort Foote near Washington, DC on the Potomac River. You can see the shape of the "mushroom" cascabel much more clearly in this photo.
Joe, We hope one of these is what you are looking for.
Mike and Tracy