The owners speak the truth. I like to underbore my paint and I haven't used anything bigger than a 0.679 freak insert in well over a year. The paint itself I haven't seen bigger than a 0.683 or so. I've even used my reball insert 0.670 a few times.
GOG has just released some newer smaller than 0.679 bore inserts, and there are some newer barrels being produced by other manufacturers as well, so it can't just be here in Michigan either.