What type of ammunition is primarily used with the M249 squad automatic weapon?

The M249 squad automatic weapon primarily uses 5.56mm NATO ammunition. This caliber is specifically designed for lightweight, personal firearms like the M249, which is meant for providing sustained fire support to infantry units. Utilizing 5.56mm ammunition allows for manageable recoil and allows soldiers to carry a larger quantity of rounds compared to larger calibers, making the M249 effective for suppressive fire in combat situations.

The design of the M249 aligns with the 5.56mm NATO cartridge's characteristics, such as higher velocity and accuracy at range, which enhances the weapon's overall effectiveness in a squad-level support role. The choice of this caliber also complements the standard issue rifles in the USMC, allowing for logistical standardization and ease of ammunition resupply in the field.