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Most hearing muffs/earplugs have a noise reduction rating of between 22 to 35 NRR.
OSHA says,
Impulse Noise: The standard states that exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level.
There is a table of firearm decibel levels here the full-size rifle cartridges listed ranged from 166.5 decibels to 158.9 decibels.
If one were to shoot at an indoor range, what is the minimum noise reduction rating one should use to protect one's hearing?
Despite what the extremely smart and dedicate (and if I say so myself good looking) individuals in the US military say is the maximum safe exposure, more noise reduction is always better to the extent that it does not hamper situational awareness or impact operational readiness. As a recreational shooter, this means more is better.
The big issue is that in the past decade we have become aware of what is called hidden hearing loss (i.e., hearing loss that audiologist cannot measure, but screws up listening in complex and noisy environments that people encounter every day). This hearing loss occurs at noise doses that were once thought safe by everyone.
I suggest everyone invest in a high quality pair of custom molded musician ear plugs. Foam and gel ear plugs generally provide less protection then custom molded ones and are harder to fit properly. NRR values are completely useless since everyone's ears are different and no one fits plugs correctly anyways. Custom molded plugs get around a lot of that.
In addition to the custom molded plugs, I would highly recommend using double protection where you wear ear muffs on top of the plugs.
[Edit to possibly placate the down voter]
OSHA allows a 140 dB peak level for impulsive noises. With 30 dB of attenuation, this puts you at 170 dB peak level. This is less than any gun you are going to find at an indoor range. OSHA allows for 90 dB for 8 hours with a 5 dB trading ratio for continuous noise. Assuming you are shooting for 1 hour, this means 105 dB without protection and 135 dB with 30 dB of attenuation. This is an average level so would require lots of people firing to keep that level constant. Again, the safety standards probably say you are fine with any type of reasonable protection. The EU uses a 3 dB trade, so depending on how long you shoot, you might get a full noise dose.
The key is that regardless of what the rules allow, you should attempt to keep your noise dose as low possible.
[DISCLAIMER: Strongbad speaks for himself and the views expressed in this answer are not endorsed nor approved by his employer]
