In 1862, Georgia dentist, builder, and mechanic John Gilleland raised money from a coterie of Confederate citizens in Athens, Georgia to build the chain-shot gun for a cost of $350. Cast in one piece, the gun featured side-by-side bores, each a little over 3 inches in diameter and splayed slightly outward so the shots would diverge and stretch the chain taut. The two barrels have a divergence of 3 degrees, and the cannon was designed to shoot simultaneously two cannonballs connected with a chain to “mow down the enemy somewhat as a scythe cuts wheat”. During tests, the Gilleland cannon effectively mowed down trees, tore up a cornfield, knocked down a chimney, and killed a cow. These experiments took place along Newton Bridge Road northwest of downtown Athens. None of the previously mentioned items were anywhere near the gun’s intended target.
r*ddit
The even easier fix is one barrel firing chain or bar shot. The method that had been done for at least 100 years at that point.
The only real reason for two barrels is to try and get a longer chain to be more effective against infantry formations, but at that point just load grape or canister shot and it works better and the cannon crew has an easier time of reloading.