Bi-metal copper coat thickness.
It's the old "bi-metal projectile/barrel wear" discussion again.
So per another thread about not cleaning out the copper as we (some) used to... and that some of us still do feel the requirement to strip it out... I was at the range and noticed that one of the Herter's copper coating was scraped forward by the bullet seating process. I discarded it.
I've sanded and looked at a cross section of the steel jacket rounds and while there appears to be a substantial coating of the copper it dawned on me that the main baring surface—not ogive—has probably been scraped during bullet seating to some extent. Now I'm wondering again if that copper is in fact doing a darn thing to help the barrel out?
I'm shooting .311 bullets in 7.62 x 39 with a .311 bore. Most of the brands that I've tried have been .311 or .310, even the U.S. manufactured ones (I measured them). So I'm wondering if that copper coating amounts to .003 reducing my baring surface to .308 which is too small for this bore.
Also, would there be an issue with long term storage if the copper is rubbed off and the steel case neck is in contact with the mild steel jacket in places?