What are you doing for data? As far as using the .41 Magnum bullets, I don't see any problem. In my SPEER 11th edition there is quite a bit of data using .357 revolver bullets in the .35 Remington which uses a .358 Rifle bullet. Something else to consider would be cast lead loads. Most companies make their bullets to .411" diameter but there are some that will custom size for you. If you slug your barrel and it is a true .411", then you could get your cast bullets sized to .412". You might try Missouri Bullet Co.