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Football helmet - Wikipedia
Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Helmets – The Concussion Blog
Guardian Caps Guardian Sports
Guardian Caps leave some NFL players and coaches with concerns: 'You feel like a bobblehead'
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Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN