To unlock this you need to collect all the submachineguns in the game. The majority is 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.
Machineguns in weapon crates:
MP5
MPX
SR-635
P90
PP19
MP7
PSG
9mm C1
9x19VSN
Scorpion EVO 3
Vector .45 ACP
Guns collected of bosses:
Experimento #42 (El Cerebro)
Residuos (El Pozolero)
Mendeleyev (Marcus Jensen)
UY! (El Chido)