While the U.S. Army adopted the M4 carbine in the 1990s, the U.S. Marine Corps retained their 20" barrel M16A4 rifles long afterwards, citing the increased range and effectiveness over the carbine version; officers were required to carry an M4 carbine rather than an M9 pistol, as Army officers do. Because the Marine Corps emphasizes "every Marine a rifleman", the lighter carbine was considered a suitable compromise between a rifle and a pistol. Marines with restricted mobility such as vehicle operators, or a greater need for mobility such as squad leaders, were issued M4 carbines. In 2015, the Marine Corps approved the M4 carbine for standard issue to front-line Marines, replacing the M16A4 rifle. The rifles are issued to support troops while the carbines go to the front-line Marines, in a reversal of the traditional roles of "rifles for the front line, carbines for the rear".