When positioned behind armor and facing gunfire, try to position yourself so you are at a 30-degree “angle of obliquity” to the shooter. Whether using a shield or an armored vehicle, bullets are more likely to be deflected when they can’t hit you straight on--which is called zero degrees obliquity. In other words, when you are positioned 90 degrees to the threat, you are maximizing the penetrating capabilities of the projectile. Positioning yourself 30 degrees off to the side will provide much better resistance.
[Armor-related Police1 column: Driving straight into the Kill Zone on purpose? No problem!]