So if I am right and the gun is on the opposite wing you will need to do the following.
In your data.ini.....
Find the section on your Left Wing:
Add:
SystemName[013]=InternalGun2
Scroll on down to your Gun
Add:
[InternalGun2]
SystemType=FIXED_GUN
GunTypeName=30MM_ADEN4
InputName=FIRE_PRIMARY_GUN
MuzzlePosition=-1.727,1.156,0.232
LightPosition=-1.727,5.200,0.232
MaxAmmo=100
EjectShells=FALSE
MinExtentPosition=
MaxExtentPosition=
Give it a rip, works for me.