In Gears of War 3 for the Xbox 360, you can unlock the Friendly Fire mutator, which will allow you to damage your teammates (and them to damage you), but will also boost your experience gained by 10%.
To unlock the Friendly Fire mutator, you need to complete the campaign in 4 player co-op.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.
