Me and a squadmate were flying around in the DA, just fooling around with them. I think the rear gun may actually be modelled as a .50 caliber weapon, instead of the 7.92 weapon it should be.
It makes the .50 caliber noise when it hits you, and it seems to pack a .50 caliber punch also.
Someone else check into it for me, it seemed to act like a .50 for both me and him, but I don't have a whole lot of experience with rear guns.