The M16/M4 rifle platform, long the standard for the US Army and Marine Corps, could soon be set aside, as officials in both service branches are looking at new options for both weapons and ammunition.

Army researchers are reportedly looking at six different types of ammunition of “intermediate calibers,” according to Army Times.

Those calibers fall between the current 7.62 mm and 5.56 mm rounds and include the .260 Remington, the 6.5 Creedmoor, and the .264 USA, as well as other variants that aren’t available commercially, Army officials told the Times.

The search for alternatives for both weapons and ammo comes in response to concerns with the 5.56 mm round and about the M16/M4 platform, which has been continuously upgraded and modified since being first introduced in the 1960s.