If your charges are too light, the bullets will not slug up to fill the cylinder throats.  And because the bullet has not slugged up to make a good gas seal, the cutting occurs and that melts bullet lead in the bore.  The rule of thumb is; if your leading is near the forcing cone, it is caused from gas cutting.  You need to increase your charge or go to a softer bullet.  If it is near the muzzle, you are driving the bullets too fast for the hardness of them.  44 Man