Source: Exotic engrams; extremely rare world drops.

Lore

"Keep it clean. Your body. Your mind. Your armor. Weapons. Gear. Ammo. CRS #1: Keep it clean. Clean equals function. Clean equals focus. That's where it all starts. Skill matters. Training, practice, experience. All key ingredients. All valued assets as you balance death and glory on the battlefield. But once you're trained and ready for war. And once your gear is up to the standards of a proper, respected, walking, talking war machine. What comes next?" "1177!" "That's right. CRS #1177: Ammunition is your best friend. So what do you do?" "Load up." "Load. Up. Get yourself a weapon that spits unending fire and stock up on ammo until your shoulders slump and your back aches. Because each round is a war all its own." —Lord Shaxx educating a gathering of Titans