Recycling bronze cannon is a practice that went back hundreds of years. Iron & stone cannon balls inevitably eroded the bore of copper alloy guns. In times of crisis, as in the CSA, church bells were collected for recycling into cannon. It was a common & ordinary practice.
Today, copper alloy items are routinely recycled. All manner of steam engine parts wore out & were recast on a regular basis during the CW period. My granddaughter is an art major with a concentration in sculpture. She regularly participates in casting aluminum, bronze & iron. My point being, apart from scale, metal casting is a straightforward process.
On order to cast an 800 lb Napoleon, about 1,000 lbs of metal had to be melted & brought to temperature. Starting with clean recycled material of the correct alloy is a great advantage. Shot out or obsolete gun barrels would have been the perfect raw material.