Collecting just the weapon cases will not get you this achievement. Each weapon type has a few weapons that you receive for killing/extracting/interrogating the boss for each region. In some regions, you will deal with boss before completing all the other missions, which was the case for getting the Warhawk from Boston Reed. Also, it seems not all weapons are needed. I acquired all the weapon cases before I collected the rifles from Santeria and Reed, so it's possible you might not need every sniper from a case if you have all 3 boss weapons, but I have no proof that is the case. Below is the list of sniper rifles, the bosses that have them and the region they are in. Hope this helps.
1. Lanza Sagrada - La Santeria in Espiritu Santo **
2. The Warhawk - Boston Reed in Koani **
3. Chica Mala - La Cabra in Inca Camino (not needed for me)