Yes but mainly in the smaller sizes (6 pounder and 12 pounder). Whitworth in the larger sizes where muzzle loaders.
Though after a little google-foo I see Armstrong Rifled Guns of the era (110 and 40 pounders) were breechloading, but I read somewhere they had problems.
Besides this isn't a foreign cannon in the Leviathan, but is US made and I work on the theory that we couldn't build large breech-loading rifles at the time.
It's my landship I'll arm it as I want to.