ORIGINAL: Briman
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.
Have you slugged those bores or are you depending upon the net for your measurements? I haven't slugged my Swiss 1911, but I have two of my K31's and the both pretty darn close to .308. As far as I know Swiss GP11 ammo is loaded with a .308 bullet diameter. That's an entirely different story than the Argentine. You can't always take the name of the caliber or information on the net for facts.
Also if you have a Swede 1896 or 38 with a bore diameter of .266, you've got a very worn bore. The rifles were never manufactured with bores that large.
As far as the Springfields, you will find some .310, but the government standard was .308. At least that's fromMaj. General Julian Hatcher's book.