I have used 'new-unprimed' factory Brass right out of the bag, without sizing.

Occasionally, a case-mouth may be slightly 'bent' in shipping. I just 'true it up' with the shank of a Philip's-Head screw-driver.

I seat primers with a Lee hand-tool. Load the new cases with measured powder charges and go right to the bullet seating Die.

With Cast Bullets, you will want to 'flare' the case mouth a bit.