It'll be tough to find lead bullets in .431" for .44mag. Most will be .430". And many will be .429". Any of these will work. If your rifling is micro-groove ala' Marlin, this is nice. The biggest thing you need to watch with shooting cast lead bullets is barrel leading. Even those of us with long barrelled revolvers have to design our loads, (diameter of bullet, lube and velocity), with leading in mind. It sucks to go to the range, have the first ten shots be right where they should be, then watch accuracy fall off to terrible by the end of the session. Even with hard cast match bullets, for practice you want to keep velocities below 1,200fps else you'll be cleaning the barrel more often than you'll want. But hard cast lead alloy bullets
can be pushed harder for hunting purposes.
Slugging your barrel is the best means of knowing what size bullet to use though. Yes, .001" larger than the barrel inner cut lands is optimum. A well lubricated .430" or .431" bullet should function fine. If you can get good accuracy with the .429" bullets, then stick with them. All lead bullets should have some sort of Alox, or Beeswax or Moly lube in the lube grooves. If not, you'll leave lead deposits in the barrel as the lead melts under friction.
If you really want to chat with some veterans of bullet casting and shooting their creations, A THR member sent me to
http://castboolits.gunloads.com/ recently. I've been rewarded with a plethora of information and some really good deals on equipment from a couple members there.
-Steve