Coaxial HMG

A Coaxial HMG is a heavy machine gun mounted coaxially to a tank's main cannon.

Battlefield 3
The Coaxial HMG is a vehicle specialization in Battlefield 3. When used, the driver gets a secondary weapon for use against enemy personnel and light vehicles. Compared to the Coaxial LMG, it benefits from a much higher damage per individual bullet, and a higher accuracy, at the cost of firing at a significantly slower rate of fire. Unlike the Coaxial LMG, the HMG does not overheat, allowing for continuous fire. Another benefit of the HMG comes from the round upgrade, from the LMG's 7.62mm up to an HMG's 12.7mm. This allows the tank driver to engage enemy helicopters whereas previously with the LMG a driver could not.

The Coaxial HMG is denser than the latter so it is mounted above the main cannon rather than through the traditional smaller barrel. You can see it easily in 1st Person, its the black barrel above the larger cannon.

The HMG upgrade is available for both the MBT and IFV