The alloy has to be hard enough to grip the rifling without skidding at the velocity you are shooting. The alloy has to be soft enough to bump up from pressure to seal the gases from flame cutting the bullet as it passes through the throats and barrel. The pressure must be high enough to bump the bullet up.
If your throats are .001 or .002 above bore diameter, your bullets fit your throats, and the alloy is hard enough to grip the rifling on its journey while being soft enough to bump up and seal everything, it won't lead at all.
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Oh yea. If your bullets are undersized, softer is better until the velocity gets to be more than the alloy can handle. Then you are out of luck.
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Undersized bullets, unless they have enough pressure to bump up and fill the bore, will cause gas cutting and leading when the hot, high velocity gases "cut" lead off the bullet on its way between the bullet and the throat and/or barrel. Leading will be mostly at the throats and forcing cone, but will continue until it fills the whole barrel if you keep shooting the offending load.
Properly sized bullets (in the range of .001 to .002 over throat diameter) will not suffer from gas cutting unless they are too hard.
Bullets that are too hard will not bump up and seal the throats and barrel well, even when sized properly, causing leading.
Bullets that are too soft to hold the rifling at the velocity they are driven at, will break loose from the rifling and "skid", breaking the seal and causing leading galore all the way down the bore.
If the bullets are sized properly, you can get away with them being a little harder than needed.
If the bullets are soft enough to bump up easily with the pressure used, you can often get away with them being a hair undersized, as long as they are hard enough to hold the rifling for the trip down the bore.
Hmm....did I miss anything? Probably.
Oh yea. Bullets that do not have enough, or good enough, lube, will tend to lead towards the end of the barrel.
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(
Copied from a
couple of my
old posts. Brain is still asleep)