NFL rejects PU safety caps for regulation play, allows 'opt-in' for practice
NFL players wear Guardian Caps to reduce head trauma during training camp
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Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries : r/nfl
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Coverage of the NFL in Guardian Caps
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
Guardian Caps', NFL's Most Recent Attempt At Preventing Head Injuries