Hi, this is my first post. I'm new to airguns. I've got a Umarex 850 and a Webley & Scott Falcon which both work brilliantly, however I was recently given an RWS C225 airpistol which appears to have a faulty seal. When I attempt to insert a new CO2 bulb, it dumps all the gas immediately. I'm pretty sure the leak is at the point the bulb is pierced. I've tried silicone lube, but this makes no difference. I believe it is simple enough to change the seal, however I can't figure out how to reach it. The part labelled "Nut at Detonater" in the manual, can't be unscrewed far enough due to a projection in the frame.

I would be very grateful if anyone can tell me how to fix this.