So in the newest Betrayal DLC, a new feature has been added to Hardline called the Gun Bench. The Gun Bench can be accessed by clicking on the "Multiplayer" tab. It should be among the multiplayer options.
In the Gun Bench you start out only being able to customize 3 weapons, but the max you can customize after unlocking the other slots is 5.
The way that the customization works is that you can build a gun just like you would normally, but you can change the dot sight to whatever shape or color you want. You can also edit certain gun skins to change how bright or faded the skins are.
To get this achievement you must unlock the other 2 Gun Bench slots. The 4th slot is unlocked by completing an assignment that requires you to customize 1 weapon in the Gun Bench (although you can customize up to three at this point) and then get 100 kills with those Gun Bench guns. The 5th slot is unlocked by having the previously mentioned assignment completed, customizing 4 guns in the Gun Bench, customizing 4 outfits (which is also done in the Gun Bench) and getting 1,000 kills with Gun Bench guns.
Once you have customized a weapon in the Gun Bench, make sure you press start and save and quit.
In order to equip the Gun Bench gun that you customized, enter a match and go to whatever class the weapon is for (such as the UZI being for Mechanics only), and there will be a new option at the top of the gun selection screen that says "Custom 1", "Custom 2" etc. Choose any of your custom weapons and next time you spawn you will have the Gun Bench gun equipped. I would like to warn you though, you cannot edit the gun at all in game, only in the Gun Bench, so make sure the weapon is customized exactly how you want it. If you are not sure if a weapon is customized the way you want it or not, you can try out your customized weapons on the shooting range IN the Gun Bench.
So the first weapon I customized was the UZI which is for the Mechanic class. I built the gun like I already had it before the Gun Bench existed, meaning I did not change the sight to a different shape nor did I choose a skin that could be edited because I assumed that as long as it was a "custom" gun, that it would count my progress. Unfortunately though, upon finishing a bunch of matches I was left with a whopping 0 kills towards the assignment.
I went back into the Gun Bench and changed the skin on the gun to something that is customizable and just moved the "Faded" and "Brightness" bars a little bit from their starting points, and then my kills started counting for the assignment. So make sure that you do something to the gun that is unique to the Gun Bench or the kills are likely not to count. By unique, again I mean either changing the shape of your sight or changing the brightness or fading on a skin that is customizable. The game will tell you if a skin is customizable or not, so there should be no mystery there.
Once you have unlocked the 4th Gun Bench slot, make sure you have 4 guns and outfits customized in the Gun Bench and then you need to get 1000 kills with Gun Bench guns. Once you have done that, the 5th Gun Bench slot will be unlocked. Customize the 5th weapon in the Gun Bench and then simply enter a game and the achievement will unlock.
If you have any questions or concerns, please let me know in the comments.