Increase weapon damage until all of your ammo runs out
Take out your pistol, fire one shot, and switch to your shotgun. The moment you switch to your shotgun, hold RB until you get the reload screen on your shotgun. Get an active reload and all of your weapons (and not just your shotgun) will have bonus damage permanently. This can be repeated. However, you must make sure that your pistol cannot be reloaded (i.e., the ammo in your clip is full).