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The one bullet at a time thing is situational. For example, if I am competing I use the magazine so I can simply work the bolt and load a new round. Magazines empty though and so I generally have a couple rounds in a round-holder stick right next to the ejection port which gives me a couple extra rounds I can single-feed into the rifle if I need an extra shot or two to complete a stage as it can be a little quicker to do than remove and reload a new magazine. In the field it's unlikely that would happen in any situation except the most dire. Also, magazines fail at times and one will simply single feed from there to keep up one's operational tempo. There's other reasons though.