The way you measure bore dimensions is to drive a lead slug through the barrel. The slug is measured in grooves and on lands. The bullet for your revolver should drop freely from the chamber thru the forcing end of the cylinders chamber.
i have seen that 2400 is very popular for magnum cartriges so maybe i should give it a shot. was hoping to keep things simple as far as the powders that i keep in stock (unique and i4198 for my .223s) but maybe i'll consider adding 2400 as well. looks like i'll be casting .357/.38 also. does 2400 work well with those?