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"The bad guys are always lousy shots in the movies. Three villains with Uzis will go after the hero, spraying thousands of rounds which miss him, after which he picks them off with a handgun."
--Roger Ebert
The forces of evil might be trained soldiers, expert marksmen, or others who kill professionally and without pity. But when they go up against the designated hero, the evil henchmen suddenly become horrible shots. This bad marksmanship is often inversely proportional to the hero's aim, as he can usually shoot from the hip with perfect accuracy. Wikipedia refers to it as the Principle of Evil Marksmanship. TV Tropes calls it the Imperial Stormtrooper Marksmanship Academy after the white-suited Stormtroopers of Star Wars fame who could never seem to hit the heroes no matter how often or how well they shot at them.
In any case, if the movie involves a decent amount of gunfighting, there's a good chance the bad guys will miss no matter how many of them there are, or what kind of weapon they are carrying. As you might guess, this cliche is invoked often because if the enemy's trained soldiers actually fired the way they were advertised, the movie would be over pretty quickly. Unlike melee combat where a blade or a hand can be blocked, parried, or twisted around, bullets cannot be told to go back in the gun once they are fired, so the solution is to make the enemy incredibly bad shots. If they do happen to hit the good guy, there's a good chance that it will be a non-fatal wound and the hero will recover quickly from it.