npas kit for gas blowback

what kinda rifle are you ordering? there's 2 main platforms, the WE and the WA platform. The WA is the more common one produced by western arms, AGM, G&P, KWA, VFC, Innokatsu and KJW etc with some minor differences between the workings of them,
The WE is a separate system of their own design with guns like the Scar Light, KAC PDW, H&K 416, M4, M16 etc etc.

Currently there are only Co2 mags available for the WE system but i believe they are in the works for the WA design.
There are NPAS sytems developed for both rifles and are completely different in design, yet work in the same way, they govern the flow of gases out through the barrel by way of a small screw that is opened and closed with the included NPAS key, sort of like an allen key.

you can order them direct from RATECH's website i believe, but ive no experience with dealing with them yet, if its the WA based system you are after then there is also a chinese copy of the NPAS system available from ebaybanned, ehobby, etc etc at a fraction of the cost of the RATECH one.