You need to think about how you are going to join the dissimilar metals.
You cannot solder, braze, or weld aluminum to copper or iron/steel.
Welding aluminum to aluminum requires special skills and tools.
Epoxy can work very well if done properly.
I find copper to be easy to work with and an excellent choice for a building material. Solder is my preferred method of joining it.
Don't bother brazing copper. The temperatures required to braze copper actually anneal the metal, and lower the pressure rating of the tube. Stick with solder and flux.
To put it plainly, the tube will rupture long before a properly soldered joint fails(calculated failure pressure), even with 50/50 lead/antimony solder.
There are several excellent copper piston guns in the pneumatic showcase section. Have a look, and see what you like.
If you decide to work with copper, download the copper tube handbook and give it a read.
http://www.copper.org/publications/pub_ ... ndbook.pdf
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