Final answer:
The caliber of the M16A4 / M4 Carbine is 5.56x45 mm NATO.
Step-by-step explanation:
The caliber of the M16A4 / M4 Carbine is 5.56x45 mm NATO, which means that the bullet diameter is 5.56 mm, and the cartridge has a length of 45 mm. Caliber refers to the diameter of the bullet or the inside diameter of the barrel. The M16A4 / M4 Carbine is chambered for the 5.56x45 mm NATO caliber ammunition, which is commonly used by the military.