What assembly serves as the support for all major components on the M249 AR?

The receiver serves as the support for all major components on the M249 Automatic Rifle (AR). It is the central part of the weapon that houses the firing mechanism and is essential for the assembly of other components such as the barrel, stock, and grip. The receiver is responsible for providing structural integrity and ensuring that all parts work in unison during operation. This component is crucial for the reliability and functioning of the weapon system under various conditions, making it a fundamental aspect of the M249 design.

In contrast, other components like the barrel, stock, and grip have specific functions and are attached to the receiver, but they do not provide the foundational support for the assembly as a whole. Hence, understanding the role of the receiver is vital for recognizing how the M249 operates effectively.