Typically, lead fouling is caused in two ways:
1.) undersized bullets
2.) lead that is too hard
Correct sizing means a bullet that is .001 larger than the bore size, ie. .358 bullets in a .357 bore. This is the correct size and should not allow gasses to pass and therefore melt the bullet, leaving lead in the bore.
Super hard lead can not obdurate and doesn't achieve a proper seal, again letting hot gasses pass the bullet melting it and leaving lead fouling.
I cast from straight wheel weights and the hardness appears to be just right for my applications. I shoot from .38spl to .454 Casull, I don't back off on the power level and I actually get more fps out of a lead bullet than a copper jacketed bullet for a given powder charge due to the inherent properties of lead and the additional lubrication from the lube applied during the sizing operation.
I shoot both gas check and plain base and I can honestly say that I don't get leading.
I even have a full power 180g cast GC 30-06 load - 2700fps.