I only have a dozen Mosins for a dozen years, but I hunt and do gunsmithing with a guy who has been buying Mosins since they were $7 in a barrel in the 1960s.
He says he has never seen any headspace problem with any Mosin.
I know I have not seen any.. except the ones I was rebarreling and in the process of headspacing.
I was getting pre war Tula Mosins for $55 at BIG5 in the first few months of 2002. If they are worth $300 now, that is 13% compounded annually.
I have been principally into Google and Amazon stock over that same aprox period, they have done 18% and 35% compounded annually respectively.
Mostly guns, guitars, and Gold appreciate at 3% compounded annually.
My guess is that the Mosin run up is over. AR15s doubled in 1994, but in the long run, they got back on the long term 3% exponential curve.