The Heart Of The Rifle
In an AR-15, the bolt and carrier are the “action.” The upper receiver serves to hold the barrel and give the bolt carrier a place to ride. There’s no integral relationship between the receiver and the firing mechanism. That’s different.
The bolt carrier “carries” the bolt back and forth in the upper receiver. It functions to house the bolt, sure, and also to reset the hammer. The carrier also bears the brunt of the gas pressure during operation; this pressure moves it back to cycle the firing mechanism. The bolt carrier has nothing to do with headspace. That’s all in the bolt. Using the same (checked and checked off) bolt in different carriers is accepted as “safe.”
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