Below is a copy and paste from feinwerkbau here:
https://www.feinwerkbau.de/en/service/downloads/technical-information/manuals?file=files/feinwerkbau/downloads/bedienungsanleitungen/luftpistolen/BA_LP_Mod P44.pdf Changing the compressed-air cylinder
The compressed-air cylinder may be unscrewed and exchanged at any time - even if not empty. By doing so, the small quantity of air from the intermediate chamber will escape audibly.
→ Unscrew by hand compressed-air cylinder (after
5 turns the cylinder must be unscrewed).
→ Fill compressed-air cylinder with compressed-air
(see filling procedure).
→ Check if the thread of compressed-air cylinder is still greased - otherwise grease slightly -.
→ Screw by hand compressed-air cylinder to the pi-stol. Pay attention that the thread of compressed-air cylinder is put on straight.
→ Screw in compressed-air cylinder until a resistance is felt. Turn further to surmount this resistance till a short „plob" is heared signaling the closing of the valve of the intermediate chamber.
I hope that helps as I do not have experience with this pistol.
Dave