A 51 Navy is perfectly appropriate for CAS shooting.  If you load the rifle and shotgun with blackpowder cartridges, you can shoot Frontier.  If a stage demands a reload (usually one more shot) you can load all six chambers, but cap only 5, and cap the 6th as a reload, or load and stage one chamber of a second (or third) gun, and cap it on the clock.  Lots of shooters shoot cap & ball.  (In England, that's all they are allowed to own and shoot).