Weapons ExpertG 20

You will need just about every diamond that he game has to offer in order to purchase all the weapons, manuals, and bandoliers from the gun shop. At the minimum you will need about 800 diamonds for the achievement, but as much as 1000 if you purchase everything he game has to offer. There are 221 hidden briefcase diamonds in the game. The remaining diamonds are acquired through faction story missions, buddy side missions, gun shop missions, and cellular tower missions. Between those you should have enough diamonds for the achievement.