File this under things I didn't know I needed! Ok, seriously, do people really use these? I have never heard of such a thing, but that doesn't mean they aren't used or useful I guess. Although all those reviews sound a bit fake/forced.
https://www.midwayusa.com/product/529577/caldwell-accumax-barrel-cooler-ar-15
https://www.midwayusa.com/product/529577/caldwell-accumax-barrel-cooler-ar-15