This is in your A-10_data.ini file
[internalGun]
SystemType=FIXED_GUN
GunTypeName=30MM_GAU8 <----------- Change this
InputName=FIRE_PRIMARY_GUN
MuzzlePosition=-0.0692,6.9625,-0.2493
LightPosition=-0.0692,6.9625,-0.2493
AimAngles=0.0,0.0,-1.0
MaxAmmo=1150
TracerLoading=5
EjectShells=FALSE
MinExtentPosition=
MaxExtentPosition=
[internalGun]
SystemType=FIXED_GUN
GunTypeName=30MM_GAU13 <-------------to this
InputName=FIRE_PRIMARY_GUN
MuzzlePosition=-0.0692,6.9625,-0.2493
LightPosition=-0.0692,6.9625,-0.2493
AimAngles=0.0,0.0,-1.0
MaxAmmo=1150
TracerLoading=5
EjectShells=FALSE
MinExtentPosition=
MaxExtentPosition=
[internalGun]
SystemType=FIXED_GUN
GunTypeName=30MM_ADEN4 <-------------or this
InputName=FIRE_PRIMARY_GUN
MuzzlePosition=-0.0692,6.9625,-0.2493
LightPosition=-0.0692,6.9625,-0.2493
AimAngles=0.0,0.0,-1.0
MaxAmmo=1150
TracerLoading=5
EjectShells=FALSE
MinExtentPosition=
MaxExtentPosition=
Sorry about the mix up.