To unlock this you need to collect all the assault rifles 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.
Assault rifles in weapon crates:
P416
556xi
AK-47
M4A1
MK17
TAR-21
SR3M
AK-12
AUG 43
805 Bren A2
G2
L85A2
R5 RGP
ACR
Guns collected of bosses:
His AK-47 (La Yuri and El Polito)
Her AK-47 (La Yuri and El Polito)
M4A1 Tactical (Carl Bookhart)
Ritmo (DJ Perico)