You can change the weapons editing handling.meta.
With Open IV in edit mode, go to:
\GTA V\update\x64\dlcpacks\jamesb\dlc.rpf\common\data\
Copy to your pc the handling.meta file and edit with notepad.
Change the weapons and save.
Replace the modified file with OPen IV.


Only can use two weapons.
order:
1- turret weapon
2- chassis weapon

Find the lines:

          <uWeaponHash>
            <Item>VEHICLE_WEAPON_TANK</Item>
            <Item>VEHICLE_WEAPON_PLAYER_LAZER</Item>
            <Item />                               
          </uWeaponHash>

And replace the weapon by another of the list:

 Available weapons:

VEHICLE_WEAPON_TANK
VEHICLE_WEAPON_SPACE_ROCKET
VEHICLE_WEAPON_PLANE_ROCKET
VEHICLE_WEAPON_PLAYER_LASER
VEHICLE_WEAPON_ENEMY_LASER
VEHICLE_WEAPON_PLAYER_BULLET
VEHICLE_WEAPON_PLAYER_BUZZARD
VEHICLE_WEAPON_PLAYER_HUNTER
VEHICLE_WEAPON_PLAYER_LAZER
VEHICLE_WEAPON_SEARCHLIGHT
VEHICLE_WEAPON_WATER_CANNON
VEHICLE_WEAPON_RADAR
VEHICLE_WEAPON_DUNE
VEHICLE_WEAPON_NOSE_TURRET_VALKYRIE
VEHICLE_WEAPON_TURRET_VALKYRIE
VEHICLE_WEAPON_TURRET_TECHNICAL
VEHICLE_WEAPON_PLAYER_SAVAGE
VEHICLE_WEAPON_TURRET_INSURGENT

And if you want change the weapon control:
WeaponSeats
Seat number that can control the weapon. Use 0 if a weapon isn't set.

Value: Weapon 1: 0 - 8. 0 is the driver.
Value: Weapon 2: 0 - 8. 0 is the driver.
Value: Weapon 3: 0 - 8. 0 is the driver.