In my limited experience, for good accuracy and lack of leading the bullet should be the same dia. or slightly smaller (.001") than cylinder throats and larger than bore size (.002" or .003"). For example, my Ruger has throats that run about .431", the bore slugs at .429", so I shoot .431" bullets and get good (for me) accuracy and not much leading. I slugged all my revolvers' cylinder throats and barrels and recorded the measurements so I can size accordingly (My Dan Wesson and Contender uses .430", my 629 uses .431", and my Puma needs .432"+). One thousandth of an inch tollerances may seem anal, but it works for me...