The start of each mission will display any unique weapons that are available. There are thirteen different weapons in the main campaign. The "Bulgarian Colossus" has a unique weapon that is equipped on the henchman available there, but does not count towards this total. The unique weapons in The Japanese Incentive DLC also do not count. If you missed any of the weapons in the main campaign, you can replay the missions if you saved the game before a particular mission. However, the weapons in the other DLCs will qualify for "Weapons Collector". You should have an autosave just before the last mission. If you purchase any of the DLCs then load that autosave, you should see the new missions on the map and be able to collect its weapon.