Look into
McMaster part number 8585K21 for barrel stock.
That's the pipe I used on my muti-barrel tee shirt launcher. Although it is on the expensive side, I had to guarantee that the tee shirt launcher would be safe to operate around large numbers of people outside, and at temperatures which had a good chance of being below freezing.
There was absolutely no PVC used anywhere on my launcher for safety reasons; everything that held pressure was rated for high pressure gas use and below freezing operation.