Which gauge kit is employed to test barrel erosion on the M2 machine gun?

The appropriate gauge kit for testing barrel erosion on the M2 machine gun is the M2A1 gauge kit. This kit is specifically designed for the M2 machine gun and allows armorers to accurately assess the condition of the barrel, particularly in relation to erosion which affects the weapon's accuracy and reliability.

Other gauge kits listed, such as those for the M16A1, M240B, and M249 SAW, are tailored for their respective weapons and would not be suitable for assessing the barrel condition of the M2 machine gun. Each weapon system has its unique specifications and maintenance requirements, which is why using the correct gauge kit is essential for proper assessment and ensuring the longevity and performance of the weapon. The M2A1 kit is specifically calibrated for the barrel measurements needed for the M2, making it the correct choice for this task.