275.00 USD - Will increase the velocity of a factory Standard gun from 700 fps with a 192 grain .500 round ball to 850 fps and fill pressure can be increased to 250 bar with no valve lock With a 450 gn slug 450 foot pounds
www.africanairordnance.com
But theres your missing power. Helium is temporary and expensive. A valve job is permanent and always at hand ;-)
The Cap is already some kind of modded gun for extra power. I would suggest getting an Air Force or Umarex Hammer or maybe AEA Zeus for more power. A tank of welding grade helium (not balloon helium) is probably going to be $200+ for a small tank and not sure how fast you will go through it. WOuld only take 5 of those to add up to a brand new Hammer or Zeus, much less on the used market since I've seen Hammers go for $450 and I bought mine for $550 (shipped) here on the forum.