51 44
As Gaucho said, the 1860 Army Colt is basically an 1851 Navy with a different barrel and larger gripframe. Colt, by the time he made the 44 Army had gotten better steel from Sheffield, England and was able to make a cylinder small enough in diameter to enable adapting the Navy frame to it by cutting a rebate in the floor if the cylinder window. This allowed a revolver of almost half the weight of the Dragoon.
Today's replicas are of even better steel than Colt could get in the 19th Century. The revolver is safe, as mentioned, with any load safe for the 1860 Army, which means, all the 3fg powder to can stuff in the chambers and still seat a ball. (about 30 to 32 gr). However, the best accuracy would be found at a bit smaller charge, 25 to 27 gr is good with a 454 ball.
Pietta's "44 Navy" is an 1860 Army with Navy grips and a bored out octagonal Navy barrel. They made it because they could, and it apparently sells well. The largest 51 Navy Colt ever made was 40 caliber, and was never produced for sale.