Install a modification of each category on a single weapon.
For this achievement you must fully modify a single weapon with a non-standard attachment in each category. The attachment categories are:

Stock
Barrel
Sight
Magazine
Gadget
You will find attachments for all weapons as you progress through the game. Whenever you kill an enemy that drops a gun, you can hold 'B' to scavenge any attachments on it that you don't currently have in your inventory. You will have no issues in acquiring at least one attachment for each category after having played through the game.

Note that the Gatling cannot be upgraded in any way, but it's very likely you will have this achievement well before ever finding a Gatling anyway.