Cock on Close
Also: Cock on Lock
An older bolt design that compresses the mainspring as the bolt is closed into battery, easing the lift required to open the action.
In a cock-on-close action the mainspring is loaded as the bolt is pushed forward and turned down into battery, rather than during the lift. The Lee-Enfield and the M1917 Enfield are the classic examples, and the design was favored for a fast, light bolt lift that let a trained soldier work the action rapidly. The energy that sets the cocking-piece goes into the closing stroke instead of the opening one.
The tradeoff is that the final push to close meets rising spring pressure, which some shooters find less smooth when chambering a tight round. Modern precision rifles almost universally use cock on open instead, because the cleaner close aids consistent feeding and lets the firing-pin state be judged easily by feel during the lift.