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Billet can be anything that is machined from one piece. It may be a billet of forged material?
A billet is a solid block of aluminum (or any material) with the size depending on the size of your...
Just using a die and taping can and usually does give a crooked thread. It should be done in a lathe with the tail stock used to support the die while the barrel is in the lathe chuck and centered. Should be a 4 jaw chuck with a spider at the other end of the spindle.
Only way to guarantee...