Some of the louder guns are going to cause damage even with muffs. Attenuation levels of 20-30 db but volume levels of 160+ and even the best muffs may not be enough.
I think it is important to take other measures like paying significant attention to having a quiet rifle. I strive for non magnum practical levels of power, using bullet weight and size, so I would prefer a 308 to a 7-08. And enough barrel length, a lot of the rifles I like now come with these tactical barrel lengths like 16" Things can be done with the crown also.
To average people the DB scale is wildly misleading. A 30-06 out of a 20 inch barrel was measured at 163db. A .375 out of an 18" barrel with a brake was measured at 170db. If we were measuring that on the relative power of the pressure wave, with the 30-06 at 100, the 375 would be 600, yet on the db scale it is only a 4.3% larger number. Also, 140db is capable of causing immediate permanent damage. So with good muffs, properly instanlled (do you wear you glasses with the muffs, you are loosing a lot of protection through that hole), the 170db will still cause damage.
Also, it isn't just the volume, it is the duration. Some blasts are longer.
The buzz you hear in your ears after when you discharge a gun, is the last broadcast of the nerves you killed with that shot. If you aren't bothered by gunfire, you already blew those nerves. As Jeff Cooper once complained, gunfire is not that loud, but this dance he went to was awful. Guess why.
Protect yourself from other activities. Don't go to rock concerts. I went to see The Cure, holy mother of god they were loud. But everyday stuff is also damaging. A hammer is in the 120 range when it contacts the nailhead, it just has a very short duration.
Take up bowhunting.
