I've watched "Mag30th" on Youtube hit some gongs from 1,000 yards with his K-31, iron sights only.
A search for "420 yards" produces pretty decent videos of his, and comparisons of muzzle energy/fps for a number of guns.
My former Russian MN 44s did really lousy with old Bulgarian ammo, and my lack of real skill contributed to very large 'groups' at 50 yards. The bayonets were extended most of the time.
Maybe Czech (light blue box) or Polish etc ammo would have been better.
Are your Mosins Russian, Polish, maybe Finnish?