Seeking to reduce head contact, NFL mandates use of Guardian Cap helmet for early portion of training camp - ESPN
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries : r/nfl
Guardian Caps: Why some NFL players are wearing them in training camps
What's With Those Padded Caps NFL Players Are Wearing In Training Camp?
Why are NFL players wearing padding on helmets at training camp?
More Guardian Caps, And '0' On The Field
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
The story behind those soft-shell puffy Guardian Caps players have been wearing