One of the primary parts of the Perfect Circle patents is the claims to not only the design but the claims of manufacturing process. I am going completly by memory at this moment so bare with. I believe one of the claims is that a hemispheric shape bonds to the fin shaped tube shape, with the payload being injected in from the top of the projectile. Almost like a blow mold, with a secondary port on the inside of the air stem filling the void created. Nevertheless, when a company bypasses a patent, they do so by not infringing on at least one claim.
So to simplify if the Perfect Circle patent says.. Our Claims are 1) The projectile has a round nose and fins set at a predisposed angle 2) The product is manufactured with paint as the payload 3) The shell is made of Polystyrene. As long as the other manufacturer eliminates only one of those claims, a new product is born. Like lets make the projectile out of foam?
Tiberius did this, with the design of their blow forward spool valve. The valve is very close, if not identical to the AGD valve systems, with the exception of the shut off valve found on AGD. Tiberius eliminated the shut off valve by reducing the air port feeding the valve chamber. The port is just big enough to flow enough gas during engine cycle, however it isn't so large that you get a rush of air keeping the bolt from resetting. This is why the Tiberius guns are limited on cycles per second.
Just my two cents