G&P have come out with a Metal Front set for the TM M16 series that comes with a QD Suppressor with an inner barrel inside to increase the FPS. The front set has quad rails for all your accessory needs. Inside is its 180mm inner barrel attached to G&P's standard hop up unit which will reach the end of the outer barrel just behind the flash hider.
If you are serious about modifications and needs a cheap way to do so, this is one way to go! It'll have the length of an M4, but an inner barrel length of an M16.