I'm guessing either this picture isn't accurate with its proportions somewhere, or they were able to sit on the head differently, because many of the stormtroopers (at least in the original trilogy) were actual people wearing costumes who needed to see out.
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However, there is a good explanation for why the stormtroopers had poor aim and the rebels didn't. Check it out here: http://www.cracked.com/article_18858_the-biggest-star-wars-plot-hole-explained-by-science.html
I don't think that's even a blooper, it's literally a scene in the movie a new hope where one of them smacks his head into the frame. In later editions released the thump sound effect was added to it
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However, there is a good explanation for why the stormtroopers had poor aim and the rebels didn't. Check it out here: http://www.cracked.com/article_18858_the-biggest-star-wars-plot-hole-explained-by-science.html