7.62 mm or .300" or 30 Cal. rifle bores are all basically the same when it comes to the land to land dia. They may vary a couple thousands but the main difference is the groove to groove dia. which can run from 309.5 to .313" on average or in some cases even higher.
Using a smaller dia. bullet gives it less grip on the rifling especially if the edges of the lands are worn. It also allow for more gas blowby which can affect both velocity and accuracy.
308 dia. bullets are always going to be the safe bet for reloading but they may or may not shoot as accurately. My Mosins all have .312 groove bores and shoot the 150 gr. .311" J bullets the most accurately.