What’s the material, air cylinders are made of?

The tubes are generally engineered to have a safety factor of around 3.5:1. Meaning its designed to burst @ around 3.5 x the pressure it is intended to retain. Any material can be used provided its designed within said specs. Most common materials are High grade aluminum 2024-t3, or titanium, although many manufacturers and older rifles still use steel / chromoly. 



So unless you're familiar with calculating high pressure vessel walls that'll withstand 3k-4k psi or whatever your fill pressure is, I would inquire with someone that is.
 
The tubes are generally engineered to have a safety factor of around 3.5:1. Meaning its designed to burst @ around 3.5 x the pressure it is intended to retain. Any material can be used provided its designed within said specs. Most common materials are High grade aluminum 2024-t3, or titanium, although many manufacturers and older rifles still use steel / chromoly. 



So unless you're familiar with calculating high pressure vessel walls that'll withstand 3k-4k psi or whatever your fill pressure is, I would inquire with someone that is.

Adding a little more to this post I would also say you need to have the equipment and the knowledge to thread and fit the parts together so it safely withstand the pressures involved. If you know a shop that has the ability and knowledge to properly and safely fit the parts together then disregard my comment😉