There are only 3 pieces to the SWAT-12: Receiver, Barrel, and Stock. All these unlock once you reach Lieutenant rank, this is the required materials and cash to make all three:
- 10 Quality Aluminum
- 10 Steel
- 8 Polymer
- 4 Regular Pin
Once you have all 3, simply equip them (other parts / ammo can be at your choosing) and finish any mission to unlock this trophy.