TargetTalk

If this is not the correct place for this post, please move it.
I have one of these guns, but have not been able to shoot it yet because the gas keeps escaping.
The gas escapes as soon as the cartridge is pierced. But at least I know where it is escaping from.
The piercing probe inside the cap obviously pierces the end of the co2 cartridge as it screws down.
What happens then is that the little brass(?) knob on the outside end of the piercing probe lifts in its housing, along with the circlip and little washer, and the gas screams out of the hole.
Any ideas what I can do to fix this please?

Working on the principle of nothing ventured, nothing gained, I removed the circlip. The 2 parts separated and lo and behold there was a broken o-ring. I have replaced the o-ring and reassembled the cap, but then discovered that I'd used my last co2 cylinder. More coming but not until Wednesday.
Meanwhile, if anyone has any other suggestions, please add them.