SJS: You're on the right track by measuring the cylinder throats. You should fire bullets that are 1/1,000ths over the throats, so .429 is a good choice. As tomf52 stated, bullet hardness is less important than size.

I've shot velocities over 1,100 fps with soft bullets (20-to-1) using 1,000th over groove with excellent long range results.

If you want to be exact, slug your barrel for the groove diameter & at least make sure the bullet is over that dimension.

Good Luck, Hank M.