Ok what size is this thread because I want to install one of those tube QD fittings like this
into the bottom of the magazine so that I can interchangeably use HPA and green gas simply because I feel like it
Here's a picture of it and a link to the actual part(one on the right is from my Tanaka fittings and the thread isn't deep enough to fit into the magazine):
http://store.kwausa.com/partsmagazinem9seriesvalveset-1.aspx
O-ring sizing (AirsoftRetreat.com)
GBB Fill Valve O-rings
Part 9452K111 (AS568A-001) - $1.87 for pack of 100
Part 9452K311 (AS568A-001-1/2) - $2.82 for pack of 100
Part 9452K112 (AS568A-002) - $1.87 for pack of 100
Note: -001 is confirmed to fit TM fill valves, but unfortunately they caused filling issues so I took them off - use with caution. I haven't used -002 yet, but I suspect they can be used for larger valves (i.e. 40mm grenades). Also, the use of needlenose tweezers to stretch the O-ring is absolutely necessary - McMaster sells those as well if you don't have any already.
On a sidenote, user Karab has informed me that he managed to use -002 O-rings for his KWA M9 PTP just fine - this technically should mean it will work with other KWA magazines, but do take that advice with a grain of salt.

into the bottom of the magazine so that I can interchangeably use HPA and green gas simply because I feel like it
Here's a picture of it and a link to the actual part(one on the right is from my Tanaka fittings and the thread isn't deep enough to fit into the magazine):
http://store.kwausa.com/partsmagazinem9seriesvalveset-1.aspx

O-ring sizing (AirsoftRetreat.com)
GBB Fill Valve O-rings
Part 9452K111 (AS568A-001) - $1.87 for pack of 100
Part 9452K311 (AS568A-001-1/2) - $2.82 for pack of 100
Part 9452K112 (AS568A-002) - $1.87 for pack of 100
Note: -001 is confirmed to fit TM fill valves, but unfortunately they caused filling issues so I took them off - use with caution. I haven't used -002 yet, but I suspect they can be used for larger valves (i.e. 40mm grenades). Also, the use of needlenose tweezers to stretch the O-ring is absolutely necessary - McMaster sells those as well if you don't have any already.
On a sidenote, user Karab has informed me that he managed to use -002 O-rings for his KWA M9 PTP just fine - this technically should mean it will work with other KWA magazines, but do take that advice with a grain of salt.