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