From Madbull, these are fully licensed Daniel Defense style Aluminium outer barrels. Tough, light and high quality; Madbull makes some of the best outer barrels around and they offer them in a broad variety of lengths. A standard M16 has a 20 inch barrel while the standard M4A1 has a 14.5 inch barrel but the beauty of the M16 family is their ability to modify the length with a lot more options than that.
18 Inches of barrel is 3.5 inches longer than an M4A1 but 2 inches shorter than an M16 giving it a somewhat hybrid compromise of handling a little easier then an M16 but out ranging an M4A1. As such, this length is know as the Special Purpose Rifle or Recon Rifle length because of its use with force recon squads operating on or behind enemy lines who need effective firepower as much as high mobility.