That valve is a commercial pneumatic Quick Exhaust valve. All metal rated for 200 psi. Below an illustration how the plumbing works on the beast.
Just posted how to adapt that fire extinguisher body into an air chamber. It stores the air charge for a single Rocket Shot. It is refilled by an standard paintball air supply via remote hose. You can set where it attaches to the regulator in the first picture. You will need an 100 cubic inch volume air chamber for your anti tank weapon unless you build something exotic. I have info for those too.
Just made another post you might find interesting for your application.
That Beast is called "The Shorty" A 6 pound, 22" long Anti Tank and Bunker Busting 'Sidearm' I can crawl with and reload quickly. Hangs on my side by a shoulder strap. Swing it up, Blast that pesky tank or bunker, drop it, and continue playing with my marker.
At its current 145 psi operating pressure, it generates a 190 fps Nerf that will look like a mortar round but REACH out to about 75 yards. At 40 yds and less, it shoots about the same trajectory as a marker ball.
I plan to upgrade the operating pressure to 195 psi to get that velocity up closer to the field limit of 230 fps out of that unusually SHORT 18" barrel. Why I upgraded for that 150 psi sprinker valve a few years back.