In general, hardness affects how easily the bullet is upset by pressure. You usually want a little bit of upset to help the bullet obturate (seal off) the bore. Absent a seal, gas escapes around the bullet, cutting and splattering lead in the bore so that it builds up quickly and starts to spoil accuracy. If the bullet is too hard it upsets little, and that means that if it doesn't start out over bore diameter it will tend to remain undersized, even under pressure. If the bullet is too soft it can upset too much, filling the chamber throat or a revolver forcing cone to a degree that can actually raise pressure over what a harder bullet would give you with the same load. It also means the bullet is overly distorted and that tends to be bad for accuracy. Also, softer lead shaves off a soft easily, so bore roughness is more prone to cause leading with one if it is fired at too high a pressure.
An old rule of thumb is to multiply the BHN of your alloy by 1422 to get the minimum operating pressure for best obturation, but if your bullet is a couple thousandths oversize, then, even if too hard, it still seems to shoot.
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