Don't let the cap & ball recommended ball size get confused with the actual caliber of the gun. The Ruger bore is 450, so it shoots, with a conversion cylinder, a 45 Colt bullet. The 457 ball is larger in order to shave a ring and be a very tight fit in the chamber when seated. After seating it is shaved/squeezed much closer to the bore diameter.
So called 44's, as in old and repro Colts and Remington cap & balls are 44 caliber land diameter, and are about 451 to 454 groove diameter, so they will also convert to 45 Colt with a conversion cylinder. Most have a slightly smaller chamber diameter and so use either 451 or 454 balls, depending on the maker.