I have found that there was a significant more copper fouling in my .270Win with which I mainly shoot Hornady Interbond Bullets. In my .222Rem with which I mainly use Sierra Game King bullets there is a significant small amount of copper fouling. On a percentage basis I guess it 70% more on the Hornady Interbond Bullets. What is your experience? I base this on the amount of times I had to apply the Copper remover (Hoppe's Copper Terminator).

The difference is much more likely in the bores of the rifle barrels than in the jackets of the bullets.