We kick this around a good bit. The early rifles were .308 but had a long, tapered throat/leade and could digest .311 ammo accordingly. Ruger had a warning in their manual that called for "either STANDARD MILITARY (my caps) or factory loaded sporting x39mm ctgs manufactured in accordance with US industry practice." Some folks interpreted that to mean no .311 import ammo. Ruger does discourage corrosive ammo.
An article ran in the "Rifleman" describing the throat/leade issue and dispelled any concerns for me regards .311 ammo in the early .308 dia rifles. Ross Seifried (?) shot .311 ammo through his test rifle in a "Guns & Ammo" article when the Mini-30 first appeared too.
Mine (.308) has digested a fair quantity of steel case from Norinco and Wolfe, but I have pretty much stopped shooting import ammo through the Mini these days.
Carried mine deer hunting today! Cannot advise when the change was made to .311 barrels.
I have not experienced the fail to fire, and broken ejectors that some report.