My first mistake...do I need to pull and redo?

['ol shooter]

Hmmm, full length re-sizing should have dealt with a high shoulder. You sure you have the seating depth right for the bullet you are using? On the ones the bolt wouldn't close on, were there any bite marks on the bullets from the rifling? You could create a very serious pressure situation if the bullet is into the rifling. Let us know what you come up with.

[BruinGuy]

There were no signs of bite marks on the bullet.

I have a suspicion..the Winchester brass were very difficult to resize, and were nearly getting stuck, especially as each got closer to the case rim. The PPU pressed all the way up to the rim, while I struggled to get the Win cases into the die completely. Perhaps a mm or so of the case near the rim did not get resized? I can see that as a problem since the cartridge headspaces on the rim (or maybe just wouldn't fit in the chamber of my rifle, since these Winchester cases were range finds). I think I need to try a more robust lube than one shot for these, and make sure it is lubed down low.

I made more than one mistake, it seems. Good learning experience, though...and my rifle thankfully saved me from myself.

[Swagman00]

Only other thing I can think of is to slug your bore.

I've heard that many Mosins do not sit right at .310 for the bore (as what you will find most pills mic at). Mine came out to .3115 and I shoot .311's without any issue. Some are even more Like .312 -.313 so don't be surprised.

However, you might have that one rare .3105 bore. Who knows?

Any old small fishing weight with a dowel, hammer, and lots of bearing grease will give the answer you're looking for.