10 times to the range and I think I've got it.
I took the 300 Win Mag Mosin to the range; 8-22, 8-26, 8-28, 9-3, 9-9, 9-10, 9-16, 9-17, 9-23, and today [9-25].
1) I loaded some 300WinMag ammo after finding only 90% of the primed brass would fit into the Mosin Nagant 300WinMag.
The difference seems to be a bulge in front of the belt.
300WinMag 125 gr Nos Bal Tip Moly, 66 gr IMR 4166, 3.34" over all length [OAL]
Quickload predicts 3174 fps 47 kpsi
a) Mosin Nagant Krieger barrel 14 pounds as tested with 4 pound butt plate and bipod
I fired 7 rounds.
Fired one sighter at 50 yards
Fired one over the chono and into the dirt
1.2" 5 shot group at 100 yards [ or 0.6" 4 shot group if I could throw out one]
What does it all mean?
good enough to proceed to long range target practice for big game.
Why did things improve?
Copper fouling powder, IMR-4166, lighter bullets, lighter triggers, less wind, and me shooting better.