 The US has done experiments regarding adopting a new, slightly larger and heavier caliber such as the 6.5mm Grendel or 6.8mm Remington SPC, which are heavier and thus retain more effectiveness at lower muzzle velocities, but has for the time decided to retain the 5.56mm NATO round as standard issue.While the US Army adopted the M4 carbine in the 1990s, the US 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.Due to the Marine Corps emphasis on being riflemen, the lighter carbine was considered a suitable compromise between a rifle and a pistol.