To unlock this you need to collect all the handguns in the game. The majority are found in weapon crates throughout the map, but a handfull can only be collected by taking down bosses in various areas. Guns found in weapon crates can be modified with gun parts and accessories whereas it is not possible to modify guns collected from bosses.
Handguns in weapon crates:
P45T
SMG-11
P227
D-50
5.7 USG
M1911
P12
Skorpion
M9
Guns collected of bosses:
La Novena (El Gato)
The BFF (El Emisario)
El General (General Baro)
Lady Killer (Antonio)