All ROAs are 45 caliber, the bore is .45 caliber and the chambers are sized to seat a .457 ball. Although the Colt and Remington repros (and originals) convert to 45 Colt or Schofield, as their bores are 45, the chambers in the repro cap & ball guns are a bit smaller than the Ruger. The difference beyond that is that Ruger calls the gun a 45, and the Colt and Remingtons were called 44s.
44 was the bore diameter before rifling. 45 caliber is the groove diameter after rifling. A question of period opposed to current labeling.
As to loads,, it is a typical Ruger. It will take more than the Colt or Remington repros, the chambers are deeper, it's not quite a Dragoon, but it will take 40 gr of powder without a wad, and you will know you pulled the trigger. I shot 35 gr of 777 powder, and certainly made an impression, a load I would never try in a Colt or Remington repro. 35 gr of BP in a cartridge conversion is the equal to any 45 Blackhawk.