The purpose of a barrel blocker for airsoft is to prevent unintended shots from firing from the gun. Even if the magazine is removed from an airsoft gun there is almost always a BB left in the barrel that can be fired. The blocker goes over the end of the barrel/muzzle and captures any accidental shots. The end of the bag is nylon-reinforced to minimize any chance of BB penetration. A barrel blocker is a mandatory piece of safety equipment at most airsoft fields and is highly recommended to be used with every airsoft gun that you own in conjunction with other safety precautions!