Modern weapons through an age-of-sail gunport

What weapon from any era would best arm a ship of the line if it had to fit through existing gunports?

Twenty-one-inch torpedoes fit the dimensional constraint and would be devastating, while railguns and rotary cannon create power, stability, and support problems.

Railguns would require a power supply far beyond that of an ordinary sailing warship. A roughly two-foot-square gunport could instead pass a 21-inch torpedo, with room to avoid the risk of jamming a 24-inch weapon in the opening. Torpedoes would be devastating against wooden opponents, although destroying the enemy would also remove the ships available to capture for prize money. A 30 mm GAU-8 Avenger in each gunport is another proposal, but the sustained recoil of a full broadside raises questions about rolling. A mixed arrangement would put cannon above torpedoes on the lower gun decks. These are deliberately fanciful armament choices whose practical limits include power, recoil, clearance, and the crew's financial incentives.