Its called Lube.That particular round especially when lead, needs lube to seal the bore around the bullet to prevent blowby and leading. With copper washed it is still needed to prevent fouling due to the soft nature of the copper wash that is on the bullet. Not near as effective as a gilding metal jacket. There is a difference between gilding metal and pure copper.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TOP PREDATOR</div><div class="ubbcode-body">Thoughts on Rimfire ammo - Q & A part 1