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The way it works is combining muffs and ear canal plugs will give roughly a 4-7 db increase in NRR so if muffs are 30 NRR and the plugs are 33 NRR then you will have a 37-39 NRR.
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Correct. However it's worth keeping in mind that a 7dB reduction actually cuts the noise power getting through by about a factor of 5, so it's not really a small reduction even though it doesn't look large when expressed in dB.
Having used plugs and muffs together in the past, I can tell you that the difference between using only one or the other vs. both is considerable.
One option is to combine active hearing protection with plugs. Turn the volume all the way up on the active muffs and that will often allow the user to hear range commands through the plugs while still providing additional protection when there are loud noises.