I have read on here several times about setting Headspace with new brass and fired brass.
It seems to me neither would be a good idea, But, I have no experience.
So I thought I would reach out here and ask the fine experienced folks on this forum.
Thanks for your answers in advance. I have learned so much from the Accurate Shooter site and am still learning.
Thanks.
A lot of very experienced gun enthusiasts here will share how they do a particular step or job, one that works well with their particular business or hobby.
I'm going to take it upon myself to share the reasons WHY you may want to use one method over another, since you mention you have no experience with this step.
I am presuming you have barrel in hand, not installed in an action, and you have brass that is new and brass that is fired at least once in a different chamber.
You may want to beg, borrow or buy a headspace go-gage also, to use to compare to what you're going to see.
With barrel in hand, slide a new brass into the chamber and measure the head protrusion. Now do that with the fired brass. Does it slide in easily? Did it get stuck? Probably going to have to FL size it first, but you just learned something about that step when using fired brass.
Slide the go-gage into the chamber and measure the protrusion. Any different?
Now place the go-gage on the bolt and slide it in the chamber. Look at the gap between the bolt head and breech. This gap is the critical distance you are setting when you screw the barrel into the action. If you also have a no-go gage measure the head protrusion on that too, and again, look at the gap between the bolt head and breech with the no-go. Then repeat with the brass, new and used.
Setting the headspace on the short side will prevent some brands of off the shelf ammo from chambering, as well as your sizing dies may not size the shoulder enough before your shell holder bottoms out on the die. Factory and hand loads may experience higher than expected pressures because the volume of the chamber is reduced.
Setting the headspace on the long side may mean your sizing dies may not size the case head portion once you've got it set for the shoulder bump you want, and you'll have trouble chambering and extracting after a couple of cycles. Volume of the case will be increased once fireformed. Set it too long and cases may separate upon first firing. Seen that happen with a factory new 300 WM.
I hope this helps you understand the what and why of barrel installation. Good luck and don't be afraid to show off your handiwork!