The lead fouling comes from the gases going past the bullet and a slight amout of melting occures. Lead,as you know,will melt at a lower temp,than copper.
As far as i can recall,jacketed bullets do not all have the same thickness of jackets ! Low speed bullets,such as ones for a hand gun or low speed rifle[30-30 ect] are thinner than, say,for a 30-06 or 7mm mag[hunting bullets].
There is more to it,but,i must go to work.