Why?
Nickle plated brass is 'stiffer' than regular brass.
I set my sizing die for the brass to just clear compressed fit when bolt closed.
In all the nickle plated brass I have used over the years, I have needed to
set the die a bit lower to get the head space fit up I want.
I size to fit a specific guns chamber on my bolts and single shot guns.
Just me I guess. Maybe most everyone else sizes down further to start out.

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