Coaxial Machine Gun

Coaxial Machine Gun allows the driver to fire a forward mounted machine gun, granting the tank considerable anti-infantry capability. This is not the same as the passenger controlled turret, so don't worry about losing any extra firepower. The machine gun fires at head level in whichever direction the body of the tank is facing, so although the range of motion is limited, it is much more effective against infantry than the tank's main turret, and does not require the driver to switch seats for machine gun access. When the main gun of the tank facing forward, you CAN in fact aim the machine gun with the crosshairs. This perk is unlocked at level 40.

In Game
Although somewhat ineffective as a main weapon against tanks, it does inflict some damage, more-so than the passenger-controlled machine gun and any infantry main weapon. It's good practice to always shoot tanks you're engaging with it along with your main gun, this will help you neutralize the tank quicker and also give you an advantage over them. With precision, you might be able to quickly kill turret gunner without losing focus on the tank at hand.

Also the Machine Gun never runs out. It can overheat, just like the passenger-controlled turret, but there's no heat indicator so it is more difficult to repeatably tap the button to prevent overheating, a common tactic with the passenger turret. The key for firing this coaxial MG is the grenade-button, by default G.

Calling this a "co-axial" gun is something of a misnomer. Originally co-axial guns fired tracer ammunition ballistically matched to a tank's main gun ("co-axial" means they share the same axis of rotation and both guns always point in the same direction.) The gunner would track the movement of the tracer round in flight, and upon impact with a hard surface, it would give off a flash and a puff of smoke. Naturally anything hit by that round would also be hit by the main gun which was then immediately fired. It remains a backup to the more sophisticated laser,infra-red and other sensors used by the onboard computers to range and target for the main gun, as well as its use in engaging personnel and light vehicles. It also allowed the tank an anti-infantry defense with a 360 degree field of fire, some Russian and Japanese tanks had an extra machine gun mounred in the turret rear for this purpose.

The machine gun the perk is ACTUALLY referring to is a ball-mounted machine gun located in the front of the hull of the tank. In real life, it was operated by the tanks radio operator,this was done because in tanks like the early T34 and French Char B the number of crew was insuffiecient to use a radio effectively. This overburdened the crew and the enemy could easily take advantage of this. But as technology and tactics improved, it was found to be easier for the radio to be operated directly by the commander of the vehicle, effectively removing the radio operator from more modern tanks, including the hull machine gun.