Me thinks that there may be more than one thing in play relative to why one Mini-30 functions well with steel-cased European ammo but another doesn't.
1)....There is a difference in a SAAMI-spec 7.62X39 chamber and a CIP-spec 7.62X39 chamber. Given that, I'd have to assume that USA-made ammo is sized slightly different than European-made ammo....which in turn, suggest that the differences in the two will result in slightly different head spacing when used in a given rifle.
2).... By design, the "tail" of the Mini-30 firing pin can not move forward when struck by the hammer unless the bolt is fully closed, which may not happen if one's ammo doesn't fully seat into one's chamber. On the other end of that mule, if the ammo sits a hair too low, the firing-pin dent will be corrospondingly too shallow.
3).,,, It has been well-documented that Ruger's manufacturing QC often aint what it should be. What that means to this discussion is that the bolt face of some Mini-30's have been found to have what looks like a "volcano shape" of displaced metal surrounding the firing pin hole which should have been removed during manufacture but was not. The effect of that is...it's a given that any amount of displaced metal will hold the cartridge away from the bolt face, so everhow "tall" that "volcano" is is exactly how much firing pin protrusion is missing relative to how much is available to hit the primer.
4)....If 1-3 above aint enough to chew on, there's also this:
Repotedly, Ruger uses some sort of "one off" chamber reamer for their 7.62X39 rifles which they loosely copied from a propiretary Lapua(sp?) design, which itself was slightly different form the European CIP standard. That was suppossedly done in order to keep the chamber pressure within the "SAAMI ball park" back when they was marketing .308 barreled rifles for use with .310-.312 ammo.
To me, what all this suggest is......
1)....There's reasons why Ruger only recomends ammo that is made to US specs.
2)....Like it or not, the Ruger Mini is not a precision-made rifle to start with, and a brass cartidge case can be forced into a slightly "out of whack" chamber much easier than can a much-stiffer steel cartridge case. Me thinks that's why brass CIP ammo works perfectly fine in a Mini-30 when steel CIP ammo won't.
3)....If simply switching to a stronger hammer spring would have "fixed" the "steel ammo in the Mini-30 problem" without it leading to other problems, Ruger would have done so years ago.
Hope this helps.
DGW