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Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
Guardian Caps debut at training camps throughout NFL
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Could Guardian Caps be used during NFL games for player safety?, Pro Football Talk
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What are Guardian Caps? NFL's latest padded helmets for added safety
What Pros Wear: Guardian Caps What are Those Helmet Covers Being Worn in Practice? - What Pros Wear
NFL players wear Guardian Caps to reduce head trauma during training camp