In general terms, how well do .308 bullets perform when fired in a barrel manufactured for the Russian .311 bullets. That's only about .003" difference. Once upon a time, SP 7.62X39 and 7.62X54 ammunition was simply not available but the Moisins and SKS rifles were pouring into the U.S. I had a FFL and a basement gun shop that was legal back in the early 1980's. I pulled the .311 FMJ's and replaced them with the equal weight .308 SP's. None of the customers that shot them ever complained, and by the late '80's, you could buy factory rounds that were legal to hunt with.

Now there is the 6.5MM, .264 round vs. the 6.5MM M.C. that is actually .267 in diameter. I ran out of my 6.5 Carcano steel cored bullets pulled from Italian Milsurp and now can only locate .264 diameter 160 gr FMJ's.

I have shot both the .267 and the .264 bullets in my Carcano and the 2.67's do better. I never shot a .308 bullet in a .311 barrel

For the record, at the time, I had a letter from the ATF that stated that simply replacing bullets of equal weights was NOT "manufacturing," nor was it considered "reloading."