It's NOT your hull MG. There is a 50 cal MG that is coaxial with the main gun. You shoot it from the #2 position. Have you ever noticed the number of rounds you have in the upper left, that is the coaxial MG.
That being said, it's ok for killing drunks, but that's about it. If you lose your turret, you also lose the coaxial MG. And it has a narrow field of view just like the main gun. If you are out of main gun ammo it can knock down that last building or two.
There are other old threads on this here in the Help section.
To make it work you have to map a key or JS button for it. I haven't even bothered since AHII came out, thanks for the reminder.