I'll second (or third? or fourth?) the recommendations of the Dell and Schwarz book, and the Alte Schiebenwaffen books. That three volume set is quite expensive, but some of the most in depth and gorgeous photography of any such book I'm aware of.
Terrific pictures and loads of info.
I get soot blow by of the brass too...
I think it has more to do with pressure vs time rather than total pressure.
That is, if the peak pressure is reached early, before the bullet has moved much, it will expand the neck and seal before the neck gets all cruddy. If peak pressure is reached later, then the gas has already icked up your brass.
That's just an untested, unresearched, theory of mine...
I just keep a piece of scotch-brite pad in my ammo box, and clean them after I shoot 'em.
Paul F.