The copper jacket on a bullet is way softer than the steel in a barrel, even on an older gun. But, copper does have a higher friction coefficient than lead. If a load with a jacketed bullet didn't make it out the barrel and a lead bullet would, the difference would be small enough that you would be pushing your luck.

Copper on steel friction coefficient- .053 (.36 lubed)
Lead on iron-.43

I couldn't find lead on steel or lead lubed. But your only talking about approx 20-30% reduction in friction with lead. Significant, but, not huge and certainly not enough to believe one would make it out a barrel and the other would not. Interestingly the coefficient of friction for something static is considerably higher than that of an object in motion. A bullet is certainly in motion once it leaves the case mouth. It is big enough to effect load data.