Co2 o ring size help!

Hi all, I have seem to have lost an o ring that I’m not sure where it goes and what size. With warm weather I hooked up the co2 adapter to my 20 oz spider bottle. I know I’m missing a O ring somewhere?? Of course as i tighten it up it leaks between the bottle and adapter. Could someone out there tell me the size and where I put it to stop leaking.
One other thing when i get it working I’m shooting 18 gr JSB around 510 fps. Is this about right fps in an 18 inch barrel?? On co2. Thanks for any info. Kip 🙂