Guardian Caps NFL: What player positions are required to wear them?
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries
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Guardian Cap: What are and why the NFL asks this safety measure in helmets
NFL expands rule against misuse of helmet; Guardian Caps required in regular season, postseason
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NFL exec suggests Guardian Caps worn to protect players during practices could one day debut in games
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
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EXPLAINER: NFL using helmet caps for player safety, research
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77