First large air cannon build

[Jaeger6]

After seeing "airsoft howitzer" on YouTube, I couldn't help myself. I'm essentially copying the zeuki1 design, except I am looking to charge the pressure vessel with scuba tanks as opposed to an air compressor for sustainability away from electricity. Has anyone here ever tried doing this? If so, how did you do it? I feel like I'm over thinking it a little bit, but my thoughts were to run a regulated line from the scuba tank to the pressure vessel for consistent filling. Also, I've been told that the pressure vessels volume should be 4-5 times that of the barrel. I'll be using 3" schedule 10 pipe (3.260" ID) for the barrel (40.5") and 6" schedule 40 pipe (48"). The projectile is the large nerf vortex football (3.375"). Something tells me that a pressure chamber of this size is waaaaaay overkill. If I can get 300 yards out of the cannon on a 45 degree elevation, I'll be satisfied. The whole thing will be mounted on a carriage, I'm not worried about weight at all.

[Sear]

Hi. Maybe your question is no longer valid, but I will answer nevertheless. I'm charging my ATG from scuba tank. You need to use regulator of course. I use adjustable regulator from 0-40 bar. My HP reservoir with ~17 ounces volume can withstand 60 bar max (870 psi), therefore a suitable safety valve needs to be included in the way. I don't think that air chamber volume has to be in any defined relation to volume of the barrel. A have paintball projectiles of similar weight to small nerf that I can launch from my setup (very long barrel) over 140 meters using only about 15 bar. There is still plenty of power left in the system.