Barrel length past Mp5 length provides no significant increase in either FPS (talking maybe 1-2 FPS) or accuracy (maybe half an inch at 100 feet) You would be best to spend your money on a quality tightbore and then use the supressor for looks. It also saves you from having to swap out the cylinder or for that fact opening the gearbox.
Changing the cylinder out would be a requirement, as the BB would reach max velocity before exiting the barrel with a standard m4 cylinder. Why is that bad? It's inefficient and the BB could be slowed down when it hits the barrel while it's exiting. It can also decrease accuracy for the same reason. Obviously using a full cylinder in an M4 barrel would produce the opposite problem, the BB would nto be at max velocity before exiting teh barrel and the rush of air after the BB exits can cause accuracy issues as well.
