I thought the issue (according to Ruger) had to do with whether or not the Mini-30 would reliably ignite hard military primers. The bore on the Mini is .308 diameter. My guess is that they cut the leade a bit long (like 5.56mm chamber) so that .310 can be used safely, but the hard primers may be the problem.
As a side note, I have a Ruger 77 in 7.62x39mm. It too has problems igniting certain ammo. It has been back to the factory twice for work on the firing pin, but the problem still exists. The rifle does well with my handloads where I use CCI primers and where I neck size only. This is all sorta OK with me as my handloads are in a class beyond factory 7.62x39mm and they group around 1" at 100 yards.
I have always wondered if this issue exists because Ruger cut the chamber a bit long, or if foreign ammo isn't held to the same dimensional standards as SAAMI spec. Just look at how hard a SKS or AK firing pin strikes the primer...no ignition problems there! My Ruger just dimples the primer enough to cause it to fire (most of the time). So, I don't know. The person at Ruger I spoke with admitted that they have problems with this issue, but I never got the "why" out of him.
dvo