Mobuck purty-much nailed it; Not all so-called ".308" bullets actually measure .308", and not all ".311" bullets measure .311". Point there being that what seems to be worrying you about the purchase could very well turn out to be akin to 'picking knat-yugo out of pepper'...Another thing to consider when it comes to the Mini-30 is "which era?", meaning that it's not a big secret that the newer renditions are far better shooters than the old-school "pencil barrel" versions...And that generally speaking, the blued Mini's are most often better shooters than the stainless models...Speaking to my own experiences with over a dozen Mini-30's, the best one I worked with was a blued model that would keep most of it's shots inside a 3" circle at 200 Yds...but...It had been match conditioned and was using the best match grade ammunition that I could assemble... .308 125 BT Sierra bullets, Lapura brass, IMR 4227, Federal Match primers. No idea what exact size it's bore was, didn't matter, didn't care.
Hope this helps.
DGW