I'd say a 12.7 x 99mm NATO firing mounted machine gun (the Browning M2HB comes to mind) would indeed be difficult to balance. You'd need really, really, high damage to make it realistic in the least; almost as much as the Xbow. This would have to be able to overheat, as in the .50BMG machine gun on the chopper in the Vehicle Mod. Though i'd like to say have a modified welding/healing meter to fill the temperature-counting meter.
I'd like to suggest these numbers in relation to balance: 600 rounds of 12.7 x 99mm NATO (.50BMG), with 50 rounds at a time. You must remain mounted to fire, and it takes about 2-3 seconds to mount/dismount the turret. In relation to the Xbow, it has 9/10 of the Xbow's damage; and costs 4000 unperked and 2500-3000 perked. 3000 for Support and 2500 for Commando. The fire rate wouldn't be faster than an AK47's, so FPs and Scrakes still post a threat to players, but not much of one if you know what you're doing; just like with everything else in KF.
This was the contribution of your resident gun nut