if the maximum limit posted for a game is 350 Feet Per Second (FPS)...and it's measured with 0.20g BBs...then if your weapon is shooting faster than that then you are not allowed to use that weapon. You can shoot any velocity below the maximum...
same if the field maximum limit is posted at 400, or 450, etc...if your weapon chronies higher than that, you can't use it.
smart fields/hosts will have a chronograph on hand. If you're in doubt as to what your weapon is shooting...get it chronied.
Shooting a roughly 430-480fps rifle at close range is likely to seriously injure someone. That is not the point of the game.