Feeding
Locking, or obturating, is restraining the cartridge so that
it helps seal gasses in the chamber while pressures are high.
For pistol rounds, simple spring force
and bolt mass is sometimes enough to meet this need.
When this is not enough, various mechanisms have been devised
to directly delay opening of the breech or to add moving mass.
One way to increase the moving mass is to allow the barrel to move,
locked together with the recoiling bolt or slide.
This is called recoil operation and is described
in two different forms in the
Operating Systems section.
Higher pressures encountered in rifle rounds usually require a strong, positive method of locking the bolt to the barrel. The most common way to do this is with a bolt which rotates, causing locking lugs on the bolt to engage corresponding recesses or interrupted threads behind the chamber or in a barrel extension.
Another way to positively lock is to tip an end of the bolt up or down to engage recesses in the receiver. In other designs a separate small breech lock slides perpendicularly to the bore axis to join slots in the bolt with the receiver. In large guns such as some artillery or naval designs, or in the manually operated falling block arms such as the Sharps rifle or modern Thompson Contender arms, the entire bolt moves laterally in a strong receiver. Especially in large guns the bolt is also called a breech block.
Next Section:
Firing