Wow! I recently discovered that this is two guns in one! The outer barrel stops at 10.3 just past the front sight and continues with a 14mm(-) extension to reach 14.5Inch, which means that this is not only an M4A1, but also a Mk.18, Mod.0 if you simply screw it off. Fantastic option! I was going to buy a second Mk.18 upper, but leaning about this saved me $$$. Good stuff. Thanks again, AD!