Copper jackets, at centerfire rifle pressures will strip in the barrel. Meaning that friction at MV is melting copper into the grooves. This smooths and in essence laps the inconsistancies in the grooves, I believe that this reduces friction and makes subsequent shots more consistent and more accurate. Many rifles only require one fouling shot and then will group true to form.
Some use this argument to challenge the need for barrel break in. On my magnum rifles this principle is extreme. Too much copper builds up in the bore and accuracy diminishes, starting after about 20 rounds in a 300WBY. I spend more time removing copper than I do applying it.