Wilson Case Gauge or RCBS Precision Mic will tell your story; at least give you oal changes before and after firing. Pretty reliable way to setup your sizer as well, go to .0035 under your 2 or 3x fired case dimension. Does brass fully form to the chamber from an unfired case? Maybe. Can be pretty sure by using fired cases to gauge. Look at several. Might see some variances. You're risking trouble if you don't sample a good number of results before you rely on a number...
Might think about polishing your chamber Flitz or Bore-Paste on a chamber mop or tight patch over a chamber brush.
Nothing "wrong" with pre-sizing bulk brass. I use an FL S die and want the neck bushing effect from the start. Cases out of a bulk bag will have dented necks. Use a 30deg VLD chamfer tool with the Wilson trimmer and get more control. Think of pre-sizing as uniforming your dimensions. Probably the time to uniform the flash hole with a broach.
Might think about polishing your chamber Flitz or Bore-Paste on a chamber mop or tight patch over a chamber brush.
Nothing "wrong" with pre-sizing bulk brass. I use an FL S die and want the neck bushing effect from the start. Cases out of a bulk bag will have dented necks. Use a 30deg VLD chamfer tool with the Wilson trimmer and get more control. Think of pre-sizing as uniforming your dimensions. Probably the time to uniform the flash hole with a broach.