First and foremost, I'm new to this forum and I'm sincerely sorry if I do anything incorrectly or say anything stupid. I'm a bit new when it comes to the engineering field.

Anyhow I'm making something of a cannon (n worries, it appears to completely legal and, for the most part, safe) and cannot seem to find the specific part I want. I've decided to get a piece of tubing (aluminum 6061-T6, Outer diameter around 2'' or more and inner between 2'' and 1.75'', probably closer to 1.75'') and have someone machine it. I'm not sure if it will be strong enough though to handle both the pressure and the force of the piston flying back though (traveling back about 1.5'' into a decent rubber backstop, behind which is a malleable iron reducing coupling)(The piston will mainly consist of 2 short aluminum cylinders with grooves cut for O-rings sitting on a steel threaded rod). The maximum pressure will be about 1200PSI (on one side, at least).

Bringing me to my next question(s), how big does the outer diameter of the tube have to be in order to thread it to 2'' male NPT (at both ends)? How thick should the cylinders be (especially the back one crashing into the backstop)?
I swear I had a third one but now I forgot.

Thanks in advanced