Threaded copper fittings are kind of rare here, so I'll leave that alone.
A couple of points;
The 90 deg. elbow leading off of the chamber will hurt flow. I would change it out for a couple of street 45's.
The opening in the propane bottle/chamber is going to be a major flow restriction. It might be fine on a launcher with a smaller barrel (<3/8"), but it won't fly with a 3/4" barrel.
Brazing copper is just a waste of time for our purposes. (JMO) The temperatures required for brazing will anneal the copper, lowering the safe working pressure.
And I rather doubt most spudders have the facilities to re-temper an assembled canon, if it were possible to do so.
Soldering/sweating is the way to go.
95/5 Tin/Antimony is the best solder strength wise, but 50/50 Lead/Tin will suffice for most.
If you want to work with copper, download this, and give it a read.
http://www.copper.org/publications/pub_ ... ndbook.pdf
More interesting reading on brazing joint/lap strength calculations;
http://www.thefabricator.com/TubePipeFa ... cfm?ID=575
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