[Hectic]

Even 330fps youll be fine, a stock TM M4 shoots like 280 or somethin and performs well indoors and out, Unless you want to have a gun for indoor and one for outdoors just stick with 330fps youll do more harm then good tearing down the gearbox so often youll wind up needing a gearbox shell prety soon as the threads for the screws that hold it together will wear out prety fast especially if you have a cheaper gun like a JG Echo1 Cymay DBoys that type of thing cause it for sure wont have a cnc aluminum or stainless gearbox in it. itll be pot metal of some kind.
Stick with the 330fps save up some cash and buy a seccond gun and you wont have to worry about spring swapping.
WHat gun do you have?
Lets say you have an M4 like most guys gety for their first aeg. even if it is a CQBr type m4 you may find it to be prety akward to use effectively indoors ao you may wind up wanting to get another gun anyhow.
Been a while scince i played indoors but I know the first time i played was with an m4, that week I bought an MP5SD6 and even with the slither stock the damn long ass front end made me an easy target approching corners and such so i converted the front to accept a flash hider instead of the silencer it was a bit better but in the end i wound up mostly just using my pistol or a mac11 i later got, a pistol or a small smg like an mp7 mp9 mac10/11 mini uzi even an mp5k is far more effective indoors then prety much any aeg is its easyer to manuver easyer/faster to aim easyer to "slice the pie" (somethin you do when comming aroud a corner in a hall or approaching a door in cqb) and remain in cover at the same time. just think how far even the shortest rifle/carbine will stick out in front of you while aiming and holding it downwards as you come around a corner means your shot befor you can even raise your gun never mind aquire a target.
Dont worry too much about fps right off the bat. make sure you are not above the lowwest limit for the fields you want to play at
(ie your cqb field) and play some games you will figure out what it is you need/want and can adjust your gear accordingly at that time.