How would reliable breech-loading cannon change Royal Navy combat in the Napoleonic Wars?
Short answerThey could raise rate of fire and accuracy, but only if metallurgy made the breech safe enough not to destroy the ship.
NotesA reliable breech-loader would let crews reload without repeatedly running the cannon back and forth through the gunport. Keeping the weapon closer to its firing position would increase rate of fire and could improve accuracy by disturbing the aim less between shots. The obstacle is containment: the breech must seal safely under firing pressure and must not endanger the wooden ship or its crew. Contemporary metallurgy made muzzle-loading the dependable norm. The counterfactual therefore depends less on tactical imagination than on solving a materials and manufacturing problem ahead of its time.