NFL makes Guardian Caps mandatory for many positions through second preseason game - NBC Sports
NFL mandates extra head protection for training camp
Here's why some Patriots players are wearing caps over their helmets during training camp
CFL introduces mandatory extra helmet protection for multiple positions. As well as other new health & safety measures. : r/sports
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Guardian Caps debut at training camps throughout NFL
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Guardian Caps NFL: What player positions are required to wear them?
Guardian Caps debut at NFL training camps to reduce force from head contact
Guardian Caps NFL: What player positions are required to wear them?
Steelers debut new, safer helmets for off-season training
Guardian Cap Debuts at Training Camps League-wide