These might be two different things really...
1. Over 550mm and you're into boreup range...I think at 590 you would be in for a kit. You'd definitely have a full cylinder...LOL, don't know how that got into this...
2. Airseal (note...I've only worked on TM AUGs...so dunno if the CA one differs significantly). Aside from within the mechbox itself, there's two things that can greatly affect the airseal between the nozzle and hopup unit/rubber. The positioning of the mechbox in the stock and the assembled position of the front end.
Typically, as long as nothing is broken in the stock, it's not too hard to ensure that the mechbox is pushed/held forward securely.
With the front end assembled, check that there isn't any back/forth play that would end up moving the barrel assembly (thus the hopup/rubber) away from the mechbox/nozzle. On some of the ones I've seen the cross bolt that you'd push to the side to release the front end can get worn...allowing the front end to move around (compromizing the air seal). I think on the last one I worked on I rebuilt the engagement surface of the bolt...(might have built up the lug that it bears against and just tidied up the cross bolt...can't remember). End result was that the front end was held tight in place and FPS/consistency picked right back up.
Best of luck
(of course the simplest way to test if you need a bore up for your rifle is to try another inner barrel that's shorter....)