To unlock weapon customization, complete the following tasks:

1. Successfully complete Side Op 107: "Extract The Legendary Gunsmith". This side op mission becomes available after story Mission 13. It usually does not appear right away because there can only be a limited number of side ops per area at the same time. Thus, keep doing other side ops, especially in the Angola-Zaire Border region, until it appears.

2. Successfully complete Side Op 108: "Extract The Legendary Gunsmith Again". This side op mission should be triggered automatically after completing Side Op 107 (return to ACC, and it should be available). If it is not available, you must keep doing other side ops until it appears. It has also appear for some people after Side Ops 06 and 48 were completed.

Step 3: Successfully complete Side Op 109: "Extract The Legendary Gunsmith Yet Again". This is the last part of this quest line. It should appear after completing Side Op 108.

You can now customize weapons in the ACC from your iDroid. Go to the "Mother Base" - "Customize" - "Weapons" tab, and choose the type of weapon you want to create. To get new parts, you must develop more weapons. The parts of each developed weapon will be available for customization.