Presuming it's the long-recoil Model 300, and not a Model 300XL gas-operated gun, IIRC the feed mechanism is operated by the recoiling barrel - releasing a shell from the magazine while the barrel's recoiling backwards and puching the bolt back.
Then, as the recoil spring returns the barrel forward, and the bolt is latch in the far "back" postition (momentarily), the barrel is withdrawn OFF the fired shell (which is held by the bolt), extracting it.
Then, as the barrel travels a bit forward, it's ejector tosses the empty out of the action, leaving the bolt to go forward, camming up the shell lifter in order for the bolt to pick up the shell just released from the mag, and chamber it.
Or, it's just magic................
Why, with long recoil guns, the chamber's loaded first, the bolt closed on the loaded chamber, then the magazine filled (or not).