Locking Lugs
The bolt lugs that rotate into matching recesses in the action to lock the breech closed and contain the pressure generated when a round fires.
Locking lugs are the projecting shoulders on the bolt that rotate into mating recesses in the receiver when the handle is turned down, locking the breech against the immense rearward force of firing. They are the structural members that hold the case in the chamber while pressure peaks, so their strength and fit are central to a rifle’s safety.
Common designs use two opposed lugs, while target and magnum actions may use three, six, or interrupted-thread arrangements to spread the load and shorten bolt lift. Because the lugs are what the action bears against at the moment of firing, any uneven contact between them is lapped true so they share the load equally, and their seating contributes to consistent headspace.