Rifle Chamber Insert (Chamber Block), 5.56mm, NSN 1005-00-418-8557 (Box of 20). Price is $2.95 per chamber block, but they are not sold individually.
Used during commercial air transit to provide a visual indicator that a weapon is not able to fire. Inserts through the ejection port and clamps between the bolt face and the chamber. Held in place by the weapon's operating spring.