You could get a velocity reducer unit. Just screws onto where the flash hider is.
That was the first link I found, you can probably find them off redwolf, ehobby, etc.
That will make it an indoor and outdoor gun. Screw it on to reduce it to under 350 and you'll be good to go for indoor games and just take it off when you're playing outdoors.
Otherwise a spring change might be better if you're planning on playing more inside games. WARNING: Some fields may not allow velocity reducers and will require you to do a spring change only.