make a chamber cast, reduce dimension's by 0,15mm and lathe turn it from solid brass stock.
this will give you the best for your money. Soldering brass tubing to turned bases is only for very low power loads with black powder. it may work some time with shotgun load's but I dare to say that you will have seperate cases and that's NO fun.
the top is 20x110 HS machine canon brass but hard to find today. its like 4 bore brass making from 20x110 US Navy.

plastik cases and blackpowder isn't a good combination. the plastik will not stand the BP heat

http://www.rockymountaincartridge.com/page7.html