US Navy Policy:
Indoor range, double hearing protection ie plugs and muffs. gunshots, if you haven't noticed, are a lot louder indoors than out..
Outdoor range, single hearing protection (plugs or muffs, shooters preference) required, double hearing still recommended. Since hearing loss counts as a disability and they end up paying for that type of thing once a body accumulates enough disability, I figure they probably put in quite a lot of study into it and I'm fine with abiding by their policy.