Can tell us where you got that information? The action for a 7.62x51 is longer and more massive I would think than the 7.62x39. The understanding in the firearms community was that the mini-14 became the mini-30. I do not a have mini-30 to a make a comparison to the mini-14 action. But normally what works for the .223 will work for AK rounds. The AR15 action will easily handle the 7.62x39 and the AKM (AK47) action will readily handle the 5.56 natio with minimal modification of the bolt face and maybe a few other things. Neither of these w/o extensive redesign will handle the .308. . What I read about the failed XGI project was the question of accuracy that they were not getting. But I am not a historian of Ruger rifles and make no strong claims.A couple more memories emerged through the fog during the night..
1) The Mini-30 was originally named the "XGI" before the caliber switch.
2) You can load .308 bullets for the Mini-30, if you want. (Don't take my word for that. Do your own research. My memory isn't what it use to be.)