I am just beginning to reload my own 7mm rounds after "building" a 7mm Rem Mag rifle. I have once-fired, nickel-plated brass and I have once-fired brass that I purchased online. I have purchased everything I have read about online to start this process and I'm now the proud owner of LOTS of new, top of the line, 7mm reloading "toys." Today I started the process by cleaning the brass pieces that I was going to start with and realized that the ONLY brass that will chamber in my rifle are new rounds (Winchester Ballistic Silvertip 150gr). My once-fired rounds that I have fired through my rifle (same Silvertips) will not chamber, also my newly purchased, once-fired brass will not chamber either. I have read that I should not FL resize once-fired brass that was fired through my rifle because it has been "fire formed" to my chamber. And when I took a micrometer to the once-fired brass versus the new, out of the box round, there is a significant difference in the case-width by a few thousandths (indicating it is, in fact, fire-formed to my rifle's chamber). I'm new at this and I don't know whether I should start by attempting to resize the shoulder down a few thousandths at a time or if I should start by trimming the case mouth?? ANY help would be greatly appreciated. And I mean ANY help. I'm swimming upstream here 
