My $.02 on Mosin Nagant accuracy
About a year ago I bought a a non sniper 91/30 Mosin. The rifling looks well defined to the eye, the fired cases eject looking ok, and I'm confident a L.E.R. 7x32 scope - S&K mount combo is stably affixed. The bore slugs out at .312" groove diameter and winchester 180 gr rounds of .310" diameter all print cleanly at 100 yards. I have floated the barrel. If I concentrate I can usually shoot a 5 round group within 1" horizontal dispersion at 100 yards. The 100 yard vertical dispersion is never smaller than ~ 3" unless I fire the rifle in my (mediocre) gun vise which manages to give me ~1'" vertical dispersion but wobbles latterally and produces terrible horizontal dispersion. So I interpret this to mean that, at firing, the rifle tries to rotate in order to conserve angular momentum and unless it is strongly restrained it will easily swing through enough of an angle about its rotation axis to throw the bullet off before it leaves the barrel ~3 M.O.A in the vertical plane. If my mosin acts like this I think most of them will. It took me about 200 rounds to wake up to this analysis since unfortunately there is a monumental amount of discussion out there distracting attention from the key requirements for consistent grouping. (Aside from ensuring that bullets are symmetrical upon exiting the barrel and have consistent velocity) my experience indicates that one mainly needs to CONSISTENTLY dampen the rifle's angular rotation upon firing to achieve minimum groups.