In Gears of War 3 for the Xbox 360, you can unlock the Must Active Reload mutator, which will jam your weapon if you don’t get a perfect active reload, but will boost your experience gained by 10%.
To unlock the Must Active Reload mutator, you need to get the Silver Active Reloader medal by getting 2,500 perfect active reloads.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.
