Just make headshots increase bodyshot damage, and bodyshots increase headshot damage, and make it a stacking buff that lasts until you reload or miss. Imagine: kill a guardian with 3 headshots, and then 3 tap somebody else with all bodyshots. Or, killing somebody with all bodyshots, and then 2 tap somebody in the face. [spoiler]Your idea sounds too similar to Polaris Lance.[/spoiler]

Awful.