You can earn this with a weapon that doesn't have any attachments, only paint schemes. You will need
K 200,000 rupees to buy 6 paint schemes in total
Visit a trading post or traveling Sherpa (
K - Kyrati Rupee symbol on map)
Open the trading interface and select, Buy & Customize in the middle. Cursor over your weapon of choice (I used the AK-47), and press
to begin customization. You may have to first assign the weapon to a holster slot, if it is not your currently used weapon. Now select the Paint circle and begin buying the 6 paint schemes.
video thanks to powerpyx.com