Henry Shooter,
I have sized my bullets from .428" through .431" and finally settled on .431". Actually, I can see no difference in the group sizes. I have casted / shot 1:40 through 1:20 alloys hunting that elusive "Eureka!" group. 1:40 scatters bullets in my gun (1860) but I can see no difference in group sizes between 1:20, 1:25 and 1:30.
Anyhoo... Absolutely none of the above left the slightest trace of lead in two Uberti's, a 60 & a 66, even with bullets with quite abit of run-out. The only time I picked up lead slivers on the patches was when I tried out a mixture of wheel-weights and lead. (That was with the 66)
I firmly believe that as long as you are shooting an alloy soft enough to "bump up", and have the pressure to bump it up, (along with a good barrel) you are safe.
But, I do know what you mean. I was once loaned an Italian Bisley 45. Accuracy wasn't very good and quickly went south from there. That night during cleaning, I discovered it had a .454" bore and I had shot ww 451s in it all afternoon. You couldn't even see the riflings. I liked to have never got the lead out of it. (Some things you never forget)
As they say, your mileage might vary.
Billy