*sigh*
Do some research, bud. This questions has been asked a MILLION times. Several of those "million" threads have been within the last two months. If you browse through the AEG section, I GARUANTEE you will find something that will answer your questions.
Oh, and its a 120 spring, not a 129(Unless maybe you found some crappy 130 spring, that isn't quite 130, so they call it a 129).