Re: Resize new brass?
Yes, generally cases come headspaced at the small end of spec.
Sometimes they come "over-bumped" and firing the case is all you can do to fix it.
Factory cases can, and do, come long in the shoulder.
Running through a resizing die, can and does bring the shoulder back into spec with respect to headspace.
If you really want to depend on your ammunition, I can't imagine not doing this step.
I've seen many guns go down because of out-of-spec cases.
A case gauge, and experience, really come in handy when setting the re-sizing die.
If your dies are not set up properly to begin with, you're getting ahead of yourself and have more learning to do anyhow.
I prefer to only have guns with spec chambers so I can interchange ammunition if I want.
At this point, if my chamber had a tight neck, I would change the chamber and not the ammunition.