Re: must I full length size new brass?
New cases are made to be loaded and shot without sizing being necessary. That being said, most could use some neck work. If they are bent or dinged they can be straightened. Sometimes the bullet grip or ID is tighter than you want.
But to talk about FL sizing is really a misconception. The new cases are smaller than your chamber in all dimensions (at least hopefully and theoretically). So running them into your die will not size the case body nor can the die be adjusted down far enough to push the shoulder back. So neck sizing is all you can accomplish anyway.
Personally I will neck size all new cases with a Lee Collet die with a mandrel that is the size I want, outside neck turn to the neck thickness I want, debur the inside of the flash hole, dry lube the inside of the necks and chamfer the neck mouth inside and out. Other than that I don't do any prep.
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