Thank you for your replies. The problem desribed above occured while loading Hornady 150 gr. round nose in Winchester and Privi brass. Winchester brass is indeed very thin and soft at the mouth, Prvi is thicker, but it was annealed. Either way, i chamfer case mouths and i seat and crimp in separate operations on separate Lee dies. The press is the breechlock system, the dies go in with a quick turn. I like this because i do not have to adjust dies avery time a load a few rounds, the adjustment is consistent from loading seassion to loading session.
I experimented with 3 things: in unscewed the top of the seating die and looked to see how that 150 gr. bullet enters and fits the seating stem. As far as i could tell, there is plenty of possibility to wiggle and enter the case mouth slanted, if the neck of the case is not centrally supported.
I also loaded Oregon Trail 170 grain .310 lead bullets, after flaring the case mouths, and 125 grain jacketed Sierra HPFN bullets, both crimped separately. To my surprise, i could not see any visible off-center problems on the necks. This ammo, with 8 grains and 10 grains of Unique, shot 1/2" groups (of 10-20 shots!) at 50 yards, at 1250 and 1450 fps. respectively.
Hm. Maybe that seating die does not like that 150 Hornady bullets, or i did not adjust it well, or the neck of those cases is not supported well while seating the bullet....
Either way, i do not need hunting ammo for this rifle, just plinking/target stuff. The rifle is set with Marbles tang and Lyman front sights, it's fun.