Riots can happen in every country, almost at any time. The police uses riot gear to protect themselves from damage, right?
Well, the riot shield might protect the riot police from molotov cocktails, rocks, etc..
But did you know that in most countries, riot shields are rated good against only pistol rounds?
So any caliber shot higher than 9mm. can break the shield, and can damage the wielder of the shield. For example, if someone were to fire a Barett .50cal at an riot police officer, it would break a part from the shield, and it would wound the police officer.