Which caliber does the M2A1 .50 Caliber Machine Gun use?

The M2A1 .50 Caliber Machine Gun is specifically designed to use the .50 caliber round, which is also known as 12.7mm. This caliber is notable for its high stopping power and range, making it suitable for a variety of military applications, including offensive and defensive operations. The .50 caliber round is significantly larger than the others listed, enabling it to penetrate heavy armor and provide effective fire support against both ground and aerial targets. This distinctive characteristic solidifies its role in military operations, particularly in the cavalry.

In contrast, the other calibers mentioned are intended for different firearms and have distinct applications. The 5.56mm and 7.62mm rounds are smaller cartridges typically used in rifles and have different performance profiles. Additionally, the 40mm caliber usually refers to grenade launchers rather than machine guns, highlighting the specific design and operational requirements the M2A1 is tailored for. Thus, C is clearly the correct answer because it accurately identifies the caliber utilized by the M2A1 machine gun.