The standard bore diameter for the M39 should be .3095, so .311 bullets may very well shoot better than .308 bullets. Especially if the bore has any wear to it at all.
You'd think, but reality is completely different.
1903 Springfields tend to have grooves much closer to .310" than to .308" yet shoot .308" bullets remarkably well.
Swedish mausers, often considered to be at the summit of accuracy for surplus firearms shoot a .264" bullet through a barrel with a .266" or larger groove diameter.
Swill 1911 rifles have grooves very close to .310" and shoot .3075" bullets.