For cast bullet shooting, you want a bullet one to two thousandths over bore size. For a revolver, the easiest way to check proper size is to push a bullet through each cylinder. There should be definite resistance, and hopefully the same on each one. Try a new bullet in each, not the same one each time.
Next thing to hope, is the barrel matches the cylinder size. If it is smaller, it will shoot well. If larger, it is going to lead, and will not be accurate most likely. Sometimes this can be solved by using a softer bullet that will bump up properly.
Most commercial companies sell bullets that are WAY too hard for pistol velocities. Beware of those advertising hard cast pistol bullets. They are for those who don't understand the charecteristics of cast bullet shooting, particularly in pistol. For pistol, air cooled wheel weight alloy will handle pretty much any application. These will be around 11-14 Brinell, and are plenty for loads up to around 15-1600 fps. generally, assuming a decent lube. Pistols don't require a high quality lube for the velocities we deal with. For something like a .38 Special, pure lead will work well.
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