As far as rifles go 5.56 is about as anemic of a round as you can get and have it still be fit for purpose in a military or defense context. You can't even hunt most things with it because it's not powerful enough. They only way these rifles are causing brain damage is if you're on the wrong end of one.
They are however incredibly loud. Being close to someone shooting 556 with a brake is a really annoying experience and I would not be shocked if there were follow on effects.
