Soldiers have been taping shaving mirrors to their bayonets and using them to peek around corners since the Siege of Petersburg.
They’ve been devising various Rube Goldberg contraptions to shoot around corners or over obstructions without exposing their bodies for about as long. Those tend to not be as popular, because soldiers seldom spend enough time shooting around corners to justifying the sacrificed they make (less range, more weight, less maneuverability) in using them.