You will easily gain this with story progression, and if you're aiming to upgrade every gun.
If you do get a kill with each gun you unlock throughout the campaign, you will unlock this not too far into Graterford, PA, when you are given the "Mutator" gun.
Every weapon in your arsenal consists of 11 guns, 1 melee weapon. So 12 weapons total.
Starting from the top and going clockwise, the weapons are as follows:
- M5A2 Carbine
- HE .44 Magnum