Well ,lets see, the BC is there for a few reasons , weight ( will explain below ) , alinement of components (BCG) & there mechanisms for bolt extraction & ejection , harnessing the gas impulse from the propellent gases ( some say its a piston ). The weight is used in some ways as a timing thing for case cool down prior to extraction( this is also why there is travel in the bolt, with in the BC, when unlocking ) , if the case was still tightly adhered to the chamber walls (ignition pressure ), you would get a lot of cases stuck in the chamber & no extraction or ripped up case rims . The carrier key helps alien it all & redirect the propellent gases to the BCG. You can lighten the weight , but you have to do other things to compensate for this weight loss , adj. gas block , heaver buffer / spring . The CUP or pressure of these firearms are quite substantial & that in its self is a good reason to have a robust BCG. I tried to keep it simple & probably missed some . <dontknow>