The length is and odd one, I could not find it in a 6.03, made by a good brand anyways. I just got a 500mm and covered it with a suppressor.
Or you can go with a PSG-1 which will allow for an even longer barrel. BE WARNED THOUGH - the PSGs mechbox is different than any other, some people say its horrible, but I love it. Very easy to work with, takes 5 mins to change out a spring.
If you do this with a G3 or PSG-1, definitely upgrade your gears, motor, piston, piston head, and cylinder head. Consistency is the key, its harder to get in an AEG, but it is possible with the correct parts.