Super-cereal.
It doesn't make it louder. The trick to deadening sound isn't so much the conductivity of the material (although it's important), but the number of boundaries between the material and the air. Think of what foam earplugs are really made of, and you'll get it.
Just by blocking the ear canal somewhat, it cuts down on the sharpness of the sound. In terms of dB hitting the eardrum, I doubt if it knocks off five.
Thing is, hearing damage is one of those cumulative things. So, you might jam a couple of .223 shells in your ears and it will stop them from ringing when you shoot... but you're still getting over 85dB at your eardrum (IIRC, 85 is about where damage starts), so each shot still knocks off a tiny bit, just a smaller bit than it would without protection.
Actually, if you know a bit about hearing and how loud guns are, you'll wind up double-covering and only shooting outdoors.