i use them all the time, i load these things in all my 30 cal rifles. i never have accuracy issues and i never have experienced increased barrel degregation due to the use of mild steel under thick copper jacket.
supposably a bi metal jacket is as follows
thin steel jacket
copper washed in a chem bath up to .008 inch
.308 caliber barrels anyone? this means the lead core and the steel jacket ring in at about .300 or so + - .001. the rifling will dig down through the copper protection and maybe, maybe not barely kiss the mild steel. very unlikely to get that far.
the jacket applied is .008 or so
after the bullets are ran through what ever sizing die or press you have a .300 bullet with .008 inch worth of protection for your rifling.
actual hardness scale in brinnel scale
304 Stainless Steel
Hardened Tool Steel
comparitivly a rifle barrel is infact tool steel (common chrome moly bottom of the line barrel material is infact the same steel they use in your transmission gears and such things in your motor;TOOL STEEL) is 675 respectivly on the scale. copper jacket which your barrel is infact in contact with most is 35, we can do this all day. mild steel? 120. compared to 675 we will never be in any kind of trouble concidering how much of our rifling would contact it if at all with a properly broken in/seen a few rounds barrel.
fankfort arsnal did a test with taxpayer money with the miliary. they wanted to find out the difference in barrel wear with copper jacket bullets or bi metal jacket bullets (bullets with copper jacket .008 thick with mild steel jacket underneath). propelled with air (not gun powder) and the results. NONE AT ALL.
verdict.........powder blast and heat errosion of powder fires degrades your barrel, not bullets.
till next time