According to Lukey52 and Rockets88 from PlaystationTrophies, if you're collecting each weapon you come across, this should be popped by Chapter 7 when you use the Ripper you start out with on that chapter. You only need to collect each gun once, a total of nine, not including your default weapon that is the Rivet Gun.
Rivet Gun (Chapter 1) (Starting Weapon)
Plasma Cutter (Chapter 1)
P-Sec Pistol (Chapter 2) (Starting Weapon)
Pulse Rifle (Chapter 2)
Flame-thrower (Chapter 3)
Line Gun (Chapter 4)
Arc Welding Gun (Chapter 5)
Force Gun (Chapter 6)
Contact Beam (Chapter 6)
Ripper (Chapter 7) (Starting Weapon)