As I was unsatisfied with the height of a standard 13/3000 tank when playing with my Phantom VSC, I started looking for a smaller alternative that would meet the following criteria. First, it had to be small enough to mount to the vertical ASA on my Phantom and still allow me to crouch, crawl and generally stay out of the way when playing low. Second, the tank had to fall within the 2x2 rule so that I wouldn't have to worry about having it hydrotested every few years. And lastly, it had to be affordable. After doing a good bit of searching and emailing, I finally got my hands on one of Maxtact's 9ci HPA tanks - and it checks off each of those boxes perfectly.
The Maxtact 9ci HPA tank measures 49.2 mm x 136.5 mm without the regulator (that's 1.937" x 5.374"). So this tank comes in at just under 2" in diameter. Check.
Below, you can see how it compares in size to both a standard 3.5 oz. CO2 tank as well as a Guerrilla Air 13/3k HPA tank. It's considerably shorter than the 13/3k; and when paired with a Myth regulator, it's so close in size to the 3.5 oz. CO2 tank that I won't even have to re-cut the foam in my gun case. Also check.
And here we see it as I had originally envisioned it. Mounted on my Phantom, the gun keeps the same feel and profile as it did with the 3.5 oz. tank but now with the ease and consistency of running on HPA. I still need to test it on the field and possibly change springs (edit: it required a blue mainspring). But if I do the math and figure that a Phantom on a 13/3k tank is good for about 150 shots (Edit: 200+)
give or take, I figure that I should be able to get about 80 shots (Edit: actual shot count is at least 150+)
or so out of a 9ci tank. So playing modified stock class, this should be perfect. Check, check and check.