If you are in the Stock Korea Instant Action you would go to your Campaign>Save folder.
Find the m.ia file. Open it with Notepad or something similar.
There you will find the hardpoints and the weapons.
You need to know the WeaponID # to make changes. You can find those in the BMS Editor under the Weapons tab, the Index number is the WeaponID#.
This is Hardpoint 3 in the file:
#Loadout Hardpoint 3
1 X AGM65D
66 W3
The first line is a comment line for the specific Hardpoint, in this case 3.
Second Line is a comment line for which weapon it is, preceded by the amount of 1, the AGM-65D .
Third line is the WeaponID# of the AGM65D, this is the only field entry you will change. The W3 remains.
The WeaponID of the MK-20D is 202.
Change the 66 to 202 and save the file. If you want you can change the Second comment line to reflect it is now the MK-20D, then save.
That’s just an example of one of the HP’s changing a single weapon.
Backup your m.IA file first.
Cheers,
C9