The PolarStar  Airsoft UGS CO2 Inserts and accessories to go with the UGS are available right now at Airsoftjunkiez... "Polarstar CO2 inserts are manufactured for both the UGS and CGS stocks and have models for both the 12g and 33g CO2 cartridges in either variant. However, the inserts have more uses than just being paired with one of these stocks. Since they are equipped with the same standard ASA threads that are on a HPA tank, they can also be threaded into a conventional regulator to make an extremely compact air rig.

The only difference between the UGS and CGS inserts is the diameter of the tube. The CGS inserts are narrower since they are designed to fit inside of a CGS's Milspec buffer tube while the UGS inserts are made to the same diameter as a Milspec buffer tube. This means by changing out the stock's rail you are able to reconfigure your UGS from running the standard 13/3000 HPA tank to a low profile, Milspec buffer tube sized cylinder so you can use Milspec AR buttstocks.

Now, it is commonly known that you should not use CO2 to power an HPA engine since if liquid CO2 gets down-steam of the regulator it can cause a spike in pressure which is unsafe and can damage the sensitive components of the engine. So how is it that we're making something like this? Our CO2 inserts are specifically designed to allow you to safety run your HPA engine off of CO2. These inserts feature an integral expansion chamber, anti-siphon system, 5 micron filter and check valve to prevent liquid CO2 from entering the regulator regardless of the rifle's or air rig's orientation. As an added benefit, the internal check valve also prevents gases from escaping the expansion chamber while switching out cartridges so as to conserve as much CO2 as possible when performing a 'combat reload' and allowing several full-power shots even while when the CO2 cartridge has been removed."