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The Nightstalker takes only the 88gram cartridges threaded into the buttstock. The only way to put a 12gram cartridge in to this cavity is via an 88 gram cartridge adapter such as this one by RWS:
Unfortunately, although this RWS adapter can be installed into the cavity of the Nightstalker, is too long to allow you to put the buttstock cap back on...
I have 2 Nightstalkers, and at one time I was going to try out the check valves that Bryan and Associates sold: http://www.bryanandac.com/
This would allow you to remove the 88 gram cartridge and reserve it for another day. However, the valve will add about an inch to the length of the cartridge, and therefore you cannot install the butt plate. I was thinking of manufacturing a single 12 gram to 88 gram adapter, however you would only get a handful of shots out of it since mine are the PAL versions and the blow-back action would use quite a bit of gas.