The M16 platform has small projectiles, but there’s a lot of powder behind them.

They are pretty loud, more so if you’re to the left or, especially, right where the ejection port sits.

My guess for SEALs is the breaching charges play a big role. Carl Gustafs are notorious as well, but I don't know whether SEALs use them. US Army Special Forces do.

Edit: Yeah, I should clarify to say the M16 is more likely to cause hearing damage, but not brain damage. The concussive force isn't that bad.