The bolt carrier would be carrying the bolt guiding it within the receiver in some automatic rifles such as the AR-10. The bolt with its cartridge head is practically useless without the bolt carrier being limited to only the cartridge and any rotational element it may possess. The bold carrier group undertakes some key functions such as Feeding – On going forward, the bolt strips a round from the magazine; Chambering - The bolt inserts the round into the chamber; Locking - The bolt locks into place in the barrel extension and Firing - The sear releases the hammer, which strikes the firing pin, thereby firing the cartridge making it one of the most important parts of your rifle.  After firing, it also undertakes unlocking, extraction, ejection and cocking actions where as the Bolt Carrier Group cycles all the way back, it cocks the hammer, allowing the weapon to be fired again.