What function does the bolt carrier group serve in a semi-automatic rifle?

The bolt carrier group plays a crucial role in the operation of a semi-automatic rifle, specifically in the cycling of the action and the chambering of the next round. When a round is fired, the gas from the fired cartridge is used to push the bolt carrier group rearward. This movement ejects the spent cartridge case, and as the carrier group moves back, it also forcibly extracts the case from the chamber.

After the spent round is ejected, the bolt carrier group moves forward again, driven by the recoil spring, and in this process, it strips a new round from the magazine and chambers it into the rifle's chamber, preparing the firearm to fire again. This automatic cycling of the action allows for quick successive shots without needing to manually operate the bolt or reload after each round.

The other options describe functions that do not align with the role of the bolt carrier group in a semi-automatic rifle. The bolt carrier group is not involved in holding ammunition before firing or stabilizing the rifle during firing, nor does it direct recoil back to the shooter. Thus, it is specifically its function to cycle the action and chamber the next round that is key to the operation of the firearm.