There is a round (of ammunition) loaded into the chamber of the pistol. A pistol (semi-automatic) operates by automatically ejecting the spent casing and loading a new round into the chamber. Each shot you fire loads the next one, so you only have to pull the trigger again to continue firing. Until the magazine is empty.
(A revolver works with a group of chambers in a cylinder.)
When you carry your semi-auto pistol it is safest to carry it without any round in the chamber. To use the gun, you then need to pull the slide back to load the first round. Subsequent rounds are loaded automatically.
If you are fully loaded with a chambered round, you have the maximum number of rounds ready to go and are carrying the gun in a condition where it will fire when the trigger is pulled (and any safety devices are disabled.)
If my gun has a ten-round magazine, I can carry 11 rounds if I have one in the chamber.
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