Impact noise, like that from gunfire, is fundamentally different from constant noise, but the damage it does is based on the SPL and the time exposed to the SPL just as with constant noise. The difference is that the rate of exposure and peak levels are the controlling factors in hearing damage to impact noise. Peak levels should not exceed 140 dB without protection. The Army lists it's 9mm handgun and the M16 as producing a peak noise of 157 dB (
http://chppm-www.apgea.army.mil/hcp//NoiseLevels.aspx. The plugs and muffs combination would effectively reduce the exposure to 125 dB (assuming the unlikely ideal insertion of the plugs and the proper use of muffs). Should the plugs not be properly inserted protection can be as little as 10 dB resulting in an exposure of ~167 dB.
http://www.combatindex.com/mil_docs/pdf/std/1400/MIL-STD-1474D.pdf is the military standard for noise exposure including impact noise like gunfire. See pages 35, 36 and the table on 38 for impact noise exposure limits in use by the military. Here are the relevant tables extracted from the document. Note that noise from firearms will as a minimum be an X category noise after the hearing protection composite NRR is applied. The Y would be the category I would act on for a range session.
http://thehighroad.org/attachment.php?attachmentid=53692&d=1172121666