How to turn a Union Dahlgren into a Confederate cannon, you could use magic or a little bit of imagination.
I could imagine my Dahlgren is one of the 11" cannons aboard the USS Keokuk, commanded by A.C. Rind. The Keokuk sunk in Charleston harbor after a battle on April 7 1863, the Federals washed their hands of the ship and had a newspaper correspondent inform the readers: "At all events she is useless to the Rebels". C.S. Navy officers, after a nighttime tour, pronounced recovery of the guns "positively Impossible". Confederate army officers spurred on by General Beauregard, who excelled in unorthodox operations and had a keen eye for heavy caliber ordnance, hired a man who could do the job. The man they hired was Adolphus W. LaCoste who was known as a Jack-of-all-trades, with a reputation for accomplishing what others deemed impossible. Working only at night, and under the guns of the Union blockading fleet, LaCoste and his men recovered the two Dahlgrens, which weighed almost 16,000 lbs. each, the operation took two and a half weeks. The feat even drew praise from former enemies, for in 1893 Rhind who commanded the Keokuk, and who was now an admiral, told a New York Times reporter that he considered the accomplishment: "one of the most daring of the war, and in point of skill had probably no counterpart"
But, since it is Confederate History Month here in Virginia, i could imagine my Dahlgren is one of the 9" cannons aboard the C.S.S. Virginia. Which on March 8 1862, sortied out of Norfolk, and being impervious to Union shot, rammed and sank the USS Cumberland and then set the USS Congress on fire.