Pretty Straight Forward.
Simply upgrade a weapon with all attachments using the same brand.
*Edit* As pointed out by IrritatedPuppy; The attatchments must be first purchased, and then also 'improved'. In order to *fully upgrade*.
Weapons have 5 upgrade slots, upgrading them with 3 attachments of the same brand is the condition required to grant an additional bonus to your weapon, take it to the limit and all the attachments on your weapon with the brand to earn this achievement.
When selecting attachments to purchase each attachment option is by a different brand.
-Nadel (3 attachments of this brand adds bonus headshot damage).
-Tempo (3 attachments of this brand adds increased ammo capacity).
-Stier (3 attachments of this brand increases all inflicted damage).
You fill each attachment slot with the same brand, fully upgrading that weapon.