Fully customize a weapon with 6 customizable parts or more
To customize a weapon, you must go into your armory and choose the weapon you want to customize by pressing on it. Then you can customize 8 different parts of it, but you only need to do 6 for this achievement. You can change parts that are free, or that remove a part of the weapon and it will still count as a customized part.
The quickest and cheapest way to do this is to use the M4 and change;
Stock, Standard to Folded Standard - Free Magazine, Standard to Extended - $3,500 Sight, Handle to None - Free Under Mount, None to Foregrip - $2,500 Side Mount, None to Rail Magazine - $3,500 Barrel, Standard to Assault - $2,000