Mosin Nagant Head space

[72 usmc]

Well, if you can find primers and the bullets... i'd say reloading is fine.
I assume you do it for accuracy? Lots of other things can be done to accurize a Mosin- sort of a contradiction in terms. Yes, if you neck resize only, keep the rounds for that rifle they were fire formed in. Mark the box with the rifle's serial number so after your first stroke you remember what goes with what. None of my many Mosins are that hard on brass. But I shoot Chinese ammo from the 80s surplus in brass, otherwise it's all seel case junk I toss.

I have not seen primers in a year.
However, if an old timer, and you got a pallet full of tins 20 years ago. That is 4 -6 cents per round. Why even bother to reload.
Has the cost of surplus corrosive 7.62x54R ammo caught up to Prvi? I reload for Lebel 8mm, but some is just not worth the effort. For my British Enfields I do neck size with a neck size set, but still after 3 reloads, I toss the HXP brass.
i always found it easy just to buy a neck size die- consistency 3 die sets are nice.

and interesting old post:
7.62x54R Full Length vs Neck Only Sizer Dies? - CMP Forums