The weapon progression system in Call of Duty: Modern Warfare 3 involves leveling up guns and, in some cases, completing specific challenges. These are often straightforward score headshots with assault rifles, beat enemies in point-blank range, and other things that you usually end up doing unintentionally. However, challenges like getting Barebones Kills are a bit more confusing, especially since the game doesn't explain what Barebones Kills even are.