Always slug your barrel then buy or size your bullets .001-.002" over groove diameter and leading will be the least of your worries. GW
Not going to help if your cylinder throats are to small
. Barrels (at least from Ruger) seem to be very consistent. It is the throats that give you the most problems. I had to ream all of my .45 Colts to .452 to get the accuracy I wanted (and stop some leading in the process). Older cylinder throats had the opposite problem of being to big.... Then you need bullets that are say .454 to get good accuracy instead of the current standard of .452.... Of course you can have a constriction in your barrel that can give you leading fits too, but that is a different topic.
I do agree with you that fit is king. If you have that you can shoot a wide range of bullet hardness in your gun. But you still don't want it to soft if shooting high velocity, nor to hard when shooting low velocity. For what I shoot 12-15 BHN works fine for 700-1100fps loads (.357/.44/.45) .