We had quite a discussion in another thread a little while back about jacketed vs lead bullets, and the force required to get them through the barrel.
And interestingly, while it does take less force to drive a lead slug than a copper one, lead bullets usually get to max pressure with less powder than jacketed ones.
One theory was that pressure on the base of a lead bullet forces it to press harder on the barrel than a jacketed one. According to that, while lead is softer than copper, it actually has MORE friction in the bore.
"Can't say its true, and I won't say its not, but there's been talk...."
(extra bonus points if you can ID that quote,
)