Teams can get quite serious in milsim but duno for CQB. I guess that gets into the competition/speedsofting area 🤮
If you've been playing for a few years you should play regularly with a few people? Ask if they want to get more serious...?
Yeah, I can understand wanting to use Co2 for winter.
If you're sure its below limits then get it chrono'd at the site without saying its Co2 or use it anyway. If you get banned then you haven't lost much.
Co2 pistols tend to be hot from my experience owning one and seeing others being used.
I'm not going to complain about Co2 pistols being banned since they tend to be used at close range and can get close to 400fps
Welcome
Do lots of reading on the forum. Search for various topics and you're likely to find answers.
Research is the key to getting a good deal, not wasting money and not being disappointed...
Could help to get in there and clean it up as well as re-shim.
O-ring nozzle is good as long as it fits properly (length and not too tight)
You could also try polishing the inner barrel.
(Presume you already cleaned the barrel...)