AFAIK all mil-spec 7.62x39 bullets measure .310-311 in diameter. I have measured Russian, Chinese, Czech, and some old East German rounds and they all were .310-311.
I have also measured US made 7.62x38 by the three big ammo makers. Winchester and Federal measured .310 inch. The Remington 123 gr JSP rounds I measured ran .308 inch. I also measured some Corbon "Urban Duty" 7.62x39 loaded with Nosler 125 gr Ballistic Tips. These bullets measured at .308 inch.
I own a number of rifles chambered for the 7.62x39. By far the most accurate is a 1989 made Ruger M77 Mark II stainless "All Weather" with a 22 inch barrel that has a .308 inch groove diameter. One would suppose that .308 inch bullets would give the best accuracy in the M77. But extensive testing at 100 yards from a benchrest proved beyond any doubt to me that the .310 bullets gave better accuracy. The Corbon load grouped five rounds into a three inch group at that distance. The Winchester "white box" 123 gr FMJ ran 1.5 inches. The Federal American Eagle ran 1.75 inches. Finnish Lapua 122 gr FMJ split the difference at about 1.6 inches. A handload consisting of the Hornady .310 inch 123 gr JSP over AA # 1680 has proven to be the most accurate load I have found to date. Five shots into an even one inch as long as I let the rather slim barrel cool down between shots.
I shot all of these loads in my scoped Ruger stainless Mini-30 Ranch Rifle as well. It also had the .308 inch groove diameter barrel. The .308 inch bullet loads gave IMHO very poor accuracy in the Mini-30. Typical group size was six inches. Good enough for rapid fire blasting at short range but a waste of money IMO. The .310 inch bullet loads gave groups running between three inches (the Win white box) and four inches (the Lapua). A tack driver the Mini-30 ain't....