PolarStar Airsoft posted on their Facebook Page, the CO2 Insert for the PolarStar UGS which will be released in late July... "Introducing the CO2 Insert for the UGS. These inserts allow you to reconfigure your UGS stock from running the standard 13/3000 HPA tank to a low profile, Mil-Spec buffer tube sized cylinder so you can use Mil-Spec AR buttstocks. These were developed in parallel with the upcoming CGS and has two variants for the 12g and 33g CO2 cartridge. Just like the inserts for the CGS, these also feature an integral expansion chamber and anti-siphon system to prevent liquid CO2 from entering the regulator regardless of the rifle's orientation. These, along with the required Mil-Spec rail, are currently in production and are estimated to be shipping before the end of July. Stay tuned for updates!"