Depends
It depends on the chamber preassure of your load among other things.
If your #2 Lyman alloy is indeed 15 BHN, that puts the chamber pressure at approx 19,109-19021 psi.
You need to pick out a load that is around that pressure. Most .357 lead bullet loads in that pressue range will be in 38 S&W Special and a 158 grain bullet. The highest velocity I see in Lees Mordern Reloading 38 Special 158gr lead bullet data is 1046 fps with a pressure of16,720 psi. ,which is well within the BHN for true Lyman #2 alloy.
BUT! Your rifles chamber ,leade, barrel and barrel length, will affect the velocity, which in turn can play a major role on how fast you can push the bullet.
Sometimes a bullet that is TOO HARD will lead the bore worse than a softer bullet that obturates to seal the hot gasses from blowing by the fragile base of the bullet.