Chamber Flag
A brightly colored plug inserted in the chamber to visibly show a rifle is unloaded, required at most ranges and competition stages.
A chamber flag is a small, usually bright orange or yellow, plastic device that slides into the open chamber of a firearm to make its empty condition obvious at a glance. The flag commonly has a tail that protrudes through the ejection port so a range officer can confirm from several feet away that the bolt is back and no round is in the chamber. It is a visual proof, not a mechanical block, and it does nothing to make the rifle fire.
Most formal ranges and nearly every match require a chamber flag whenever a rifle is on a rack, slung, or otherwise not on the line being fired. It complements the safety-selector and bolt-back protocol as a redundant safety check that anyone can verify without handling the gun. Between stages, with the rifle flagged and pointed in a safe direction, a shooter can review a range-card or wind plan without any safety ambiguity.