Is there an HPA drop-in replacement for CO2?

Used to be some on the market not sure anymore.

IF you find one and use air you will get somewhere between 2 and 8 shots, fun eh?

Some poeple have used "dummy carts" a cartridge with a hole drill & fitting installed that allows hooking up to a coil line & HPA bottle - personally on the rigs you mentioned "I" would take the psi down to about 400psi) - . 

Dont have the link handy but the mil scpec acrts are about .18each purchased by the 100.

John

The big issue going to hpa is the gun and the valving are made and adjusted for the low pressure of the co2. There are bulk thethering adapters and larger co2 cylinders, but the 12 gram cartriges bought in 100+ are relatively cheap.
. You would be better off just getting a pcp instead of all the convertions. Unless you enjoy such a thing.