You just need to infuse (save between loops) each weapon, of any quality.
Standard weapons droped by all enemies
The small SMG.
The shotgun type.
The chaingun type rifle.
The single shot rifle.
(Unconfirmed if needed.)
The SMG that converts to dual pistol. Unlock the Armory in The Complex via the four battery lock.
The legendary SMG that can reload while shooting. Dropped by Frank in Fristad Rock - Morning.
The legendary shotgun from Charlie's Reward Scheme in Karl's Bay - Evening.
The legendary sniper rifle that is dropped by an assasin in Updaam - Afternoon (part of a story/weapon quest).