I think the main reason .38-40s persisted is that they managed to make the barrels very consistent, unlike the .44-40 whose dimensions wander all over the place. The same thing seems to be true of .38-40 and .44-40 revolvers. This issue became even more important when the switch to smokeless powder occurred.
Ive had .44-40 Wins that had from .426 to .432 bores, but all the .38-40s I've had were .400-.401. Colts AND Winchesters.
