Don't settle for a bulletproof vest with only one layer of shoulder strap. Ours has an inner strap and an outer one for added protection. You'll need two because you want your vest to stay on in extreme situations, and although you need a lot of velcro, you don't want the straps to be too wide or the vest won't be comfortable. We've tried other configurations and a double-strap arrangement is the best.
When putting on the vest, you fasten the inner strap first, then the outer one.
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