The first is your progression through the main story. The weapon merchant isn’t always available; he disappears for periods, and you can only unlock new guns by completing story missions. For example, the iconic AK-47, a fan-favorite primary weapon, is only unlocked after completing the second convoy mission in Act 1.
Look for a reliable source to download the Far Cry 2 all weapons unlocked save game. Be cautious with downloads from the internet, as they can sometimes contain malware. Far Cry 2 All Weapons Unlocked Save Game