Pic. CO2 Cartridge Adapter can be hidden inside most of the rifle stocks. No pneumatic tubes will be visible from outside.
Pic. Adapter with the regulator is quite long – 180mm with the MMR regulator. But slim enough to fit the stock – 25mm diameter.
Pic. It can be installed as well outside. Simple use of cable tie or U-shaped adapters for ris rails – https://mancraftuk.com/product/u-shape-13ci-0-2l-tank-adaptor