Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
NFL hopes to reduce head injuries with helmet experiment - The Sumter Item
Concussions: Can cap keep football players' heads in game? – Orange County Register
NFL says Guardian Caps reduced concussions by over 50%
49ers training camp: Pads come on, but head-on contact frowned upon
NFL players wear Guardian Caps to reduce head trauma during training camp
NFL players experiment with 'Guardian Caps' to protect heads
NFL expands rule against misuse of helmet; Guardian Caps required in regular season, postseason
NFL exec suggests Guardian Caps worn to protect players during practices could one day debut in games
Guardian Caps: What are they and why are NFL players wearing them? - Sactown Sports
EXPLAINER: NFL using helmet caps for player safety, research
Xaverian High in Mass. introduces Guardian Caps for football players
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Guardian Cap Debuts at NFL Training Camps League-wide
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries