As the name suggests the primary weapons in Far Cry 2 are very important to get through the game. These include quite an interesting variety of weapon types:
Upgrades become available once you have purchased the appropriate weapon via Online Weapon Shop, on the Manuals page. So buy the weapon, and then immediately buy the desired upgrade. All weapons, except for the IEDs feature Accuracy and Reliability Upgrades.
Accuracy Upgrade
These upgrades improve the hit precision of weapons.
Reliability Upgrade
These upgrades prolong the life of your weapons, i.e. they are less likely to jam or break down.
Leboa-Sako (Northern District)
0th
These weapons are available before you take on any 'convoy mission' via . Note: They don't become available all at once. Three of them (Homeland 37, G3, and Makarov) are unlocked when you first visit a Weapon Shop in the Leboa-Sako (Northern District) during the tutorial. Three more (M1903, PKM, and IEDs) are unlocked when the tutorial is over (after you are told that "you are free to explore the world"). The Fortune's Edition of Far Cry 2 adds three extra weapons, which don't require unlocking or purchasing and may be picked up at any time.
1st
These weapons become available at any Weapons Shop after you finished the first of four 'convoy missions' .
2nd
Finish the second of four 'convoy missions'.
3rd
Finish the third of four 'convoy missions'.
4th
Finish the fourth of four 'convoy missions'.
Bowa Seko (Southern District)
4.5th
These weapons become available after you finished the main missions in Leboa-Sako (Northern District), and are on your way to Sefapane in Bowa Seko (Southern District). They become available even if you did not do the first four 'convoy missions' () in the Northern District.
5th
These weapons become available at any Weapons Shop after you finished the first of four new 'convoy missions' ().