I wouldn’t go all out for a TC mod for simple ammo change, having said that: if you’re using the Assault Rifle (which is PrimalItem_WeaponRifle) You’ll need to edit the Weapon Class attached to it (WeapRifle).
IF you are simply trying to put some normal ammo for the original gun into the Inventory of the Box you made (like an ammo box) - I would NOT copy the ammo over and create an all new one, with the weapon, etc. etc.
Simply point the Inventory to the Core PrimalItemAmmo_AdvancedRifleBullet a bunch of times (Once for each Bullet you want in there.)
The core file is at PrimalEarth/Weapons/PrimalItemAmmo_AdvancedRifleBullet
This is what it sound like you’re actually doing. So I’d just do that step above.
If you’re creating a new craft-able bullet amount, You’ll need the following items to craft a custom gun to use your custom ammo:
The Engrams (for both your new bullet and the weapon that can fire it.) For the Assualt Gun - it’s EngramEntry_AdvancedRifleBullet and EngramEntry_MachinedRifle
The PrimalItemBPs. Which are: PrimalItemAmmo_AdvancedRifleBullet (Here’s where you change how many are crafted per try) and PrimalItem_WeaponRifle (This is to point to your WeaponClass (WeapRifle)
The WeaponClass, WeapRifle. In here you can change the default Weapon Ammo Template to point to any ammo type you wish, in this case your new PrimalItemAmmo_AdvancedRifleBullet.
I’d rename a few things to keep things clear and precise.