You need to retrieve 5 heavy weapons via Jack's Fetch ability ("Fetch Heavy" - must be opened up in Core upgrades). Easiest way to do it - collect all of them in Act 2.

Tri-shot - Act 2 - Part 1 (first fight after generator)
RL-4 Salvo - Act 2 - Part 1 (near Silverback)
Mulcher - Act 2 - Part 2 (Train tunnel)
Buzzkill - Act 2 - Part 4 ("Nethercutt" Mine)
Cryo Cannon - Act 2 - Part 5 (Laboratory, before fighting boss)