I think the issue here is that you're looking for a silent solution. So far as I've seen the TM Mk23 is the only gas gun that comes close to being silent.
However, there might be a possibility of converting a gun to be silent. I've personally been toying with that idea while using a Soccom Gear Gemtech Oasis as a base. 99$
Airsoft Evike.com- *NEW* SocomGear GemTech Oasis Pistol - YouTube