Many good/correct things have been said about noise. I was an Certified Industrial Hygienist by profession but have been retired/disabled for twenty years, so I'm not up on the latest and haven't tried to keep up.
One thing that hasn't been mentioned is there are two different types of noise, continuous and impact. The NRRs you see on hearing protection devices are for continuous noise, not impact, and gunshot noise is impact noise.
A NRR is arrived at using specified testing methods and is really useful only in comparing HPDs.
I believe the OSHA Hearing Conservation Amendment allows only one subtraction, but OSHA laws don't apply to most of us at shooting ranges. Putting muffs over plugs is effective for our purposes, tho.
It's a 3dB doubling. 30dB times two is 33dB. It's an easy calculation on modern calculators. Divide the dB by 10. Raise 10 to that power. Sum all the numbers and take the log. Multiply the log by 10. Remember, decibel means a tenth (deci) of a bel. The numbers are related to each other similar to Richter scale or pH numbers.
Hearing protection was not required when I was in the Army. We put cigarette filters in our ears when shooting the M1. I was in 105mm batteries both on active duty and the NG and hearing protection wasn't offered. The loudest thing I heard was the 90mm gun on a M48. I understand the 105mm gun on the M60 was louder.
The foam plugs are very useful, but you need to fit them properly as Oro directed and refit them thru the shooting session. I have never thought too much of muffs as hair and glasses and movement affect the fit. I'm told the electronic muffs work very well. The worst protective equipment is that which isn't used correctly. One detriment of the foam plugs is one's hands need to be clean when rolling them up or the dirt from your fingers and thumb go right down your ears.
I use the baffle type plug. My daughter, a NG major, had some and gave them to me. Peltor makes some that seem identical. I shoot up to .41mag and get good protection.
Lots of good advice in this thread. Find hearing protection that suits you and use it.
Tom