Working on some measurements and pics as I type this. Will be editing to add things.
3/4 inc pipe. I use a simple hand pipe cutter for easy/precise cuts.
This is the base which holds two guns and however many mask you want. It is only restricted because of the way I decided to glue certain pieces. (Notice the corners and how they connect to the first tank or barrel rest)
Same thing but showing it apart. Any pieces that are together here are glued.
Rough, not to scale drawing of the picture above with dimensions.
The top 4 items in the drawing are what you need to expand the rack to a certain size. They all remain the same so just duplicate them to fit your needs. The "T" fitting I used to rest the barrels on is cut in half but that's preference. Any questions let me know. I'm sure it's clear as mud but I think all the measurements are there.