In Gears of War 3 for the Xbox 360, you can unlock the Super Reload mutator, which will increase your damage and clip size after a perfect active reload, but it will also cut your experience gained by 25%.
To unlock the Super Reload mutator, you need to earn the Bronze Master-At-Arms medal by earning a Bronze medal with all five starting weapons. This is done by getting 100 kills with each starting weapon in versus matches. The five starting weapons being the Lancer, Retro Lancer, Hammerburst, Gnasher, and Sawed-Off.
This is probably easiest in private matches against bots set to casual difficulty.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.