My 22-250's shoot 40 gr bullets at 4000 fps and definitely copper-foul the barrels. My .357 rifle with cowboy loads shoots around 1000 fps and definitely does NOT lead-foul the barrel. I don't know yet whether my .17 HMR at 2500 fps will cause copper-fouling. I know it's partly a function of how smooth or rough the bore is, but do you guys have a feel for the velocity where fouling becomes a concern? Perhaps over 3000 fps?