They Blackhawk .45 convertibles I have owned have shot equally well using .45 colt and .45 ACP handloads. 2 to 2/12" groups at 25 yards.
The 9mm convertible .357 I shot seemed to be slightly less accurate using ball ammo compared to the .38/.357 reloads fired in it. That was probably due to the slightly undersized and mediocre surplus ball ammo used. 38/.357 accuracy was very good, giving 1 1/2 to 2" groups at 25 yards.