I would say approx. 30cm - 35cm, about 12 or 13 inches.
But I plan on making it a bit shorter, so it fits neatly with the design. The final barrel length should approx. be 25 cm, or 10 inches.
I see what you did there, using you air soft gun to make there barrel eh? well smart move but the cylinder on the m4 part gets bottle necked at parts rather than a strait cylinder, also it looks a wee bit long but hat could just be me.
edit
what did you use to make you pistol, my pep one is not that smooth?