Couldn't resist posting the following link.
https://trevoronthetrigger.wordpres...me-performance-the-misleading-nrr18db-rating/
I do agree a system with around 30 db NRR is a good thing to strive for. Though the numbers can be skewed as the article in the link shows.
High NRR muffs are bulky, maybe too bulky for rifle/shotgun shooting. Fine for handgun though since getting your head to a stock to see the sights isn't an issue. The highest rated muff you can get is around 31 NRR, passive only. They rely on large volume ear cups that stick out about 2-1/2". Electronic muffs will be in the low 20's NRR at the best.
Any cheapie disposable foam plug is going to be in the 28-33 NRR range. Assuming it is well inserted.
http://w ww.earplugstore.com/unfoamtrialp1.html A lot of people will not insert them far enough.
The reusable silicone type plugs like Surefire Sonic Defenders are at 24 NRR.
Custom molded avoid the user induced problem of improper insertion since they fit perfectly, usually around 30 NRR.
Currently outdoors I use MSA electronic muffs with gel seals over top of EP3 Sonic Defenders. The two together should be around 30 db NRR (NRR numbers don't add directly) The big thing for me though is I can actually still hear my surroundings with the electronic muffs and plugs due to the amplification.
For indoors I have giant passive Peltors and foam plugs available. Custom molded might be in my near future to bring my setup up a dew db and still be user friendly.