Here are some data for some common guns; it's higher than you'd expect. For long guns, there is a definite correlation with barrel length.
Here is some data. Not sure if these figures are straight dB or dBA.
http://www.freehearingtest.com/hia_gunfirenoise.shtml
Table 1. SHOTGUN NOISE DATA (DECIBEL AVERAGES)
.410 Bore 28" barrel.....150dB
26" barrel...............150.25dB
18 _" barrel.............156.30dB
20 Gauge 28" barrel......152.50dB
22" barrel...............154.75dB
12 Gauge 28" barrel......151.50dB
26" barrel...............156.10dB
18 _" barrel.............161.50dB
Dr. Krammer continues to say that shotgun noise averaged slightly more that 150dB. This is approximately 14dB beyond the threshold of pain, and more than sufficient to cause sudden hearing loss with complications.
Table 2. CENTERFIRE RIFLE DATA
.223, 55GR. Commercial load 18 _" barrel.....155.5dB
.243 in 22" barrel...........................155.9dB
.30-30 in 20" barrel.........................156.0dB
7mm Magnum in 20" barrel.....................157.5dB
.308 in 24" barrel...........................156.2dB
.30-06 in 24" barrel.........................158.5dB
.30-06 in 18 _" barrel.......................163.2dB
.375 — 18" barrel with muzzle brake...........170 dB
Krammer adds that sound pressure levels for the various pistols and ammunition tested yielded an average mean of 157.5 dB, which is greater than those previously shown for shotgun and rifle noise levels. There was also a greater range, from 152.4dB to 164.5dB, representing 12 dB difference, or more than 10 time as much acoustic energy for the top end of the pistol spectrum. It should be noticed that this figure of 164.5 dB approaches the practical limit of impulse noise measurement capability inherent in most modern sound level meters.
Table 3. CENTERFIRE PISTOL DATA
.25 ACP...........155.0 dB
.32 LONG..........152.4 dB
.32 ACP...........153.5 dB
.380..............157.7 dB
9mm...............159.8 dB
.38 S&W...........153.5 dB
.38 Spl...........156.3 dB
.357 Magnum.......164.3 dB
.41 Magnum........163.2 dB
.44 Spl...........155.9 dB
.45 ACP...........157.0 dB
.45 COLT..........154.7 dB
For those who don't grok decibels, it's a logarithmic scale (usually log10); a 3dB difference equals twice the radiated acoustic energy, and a 10dB difference is ten times the radiated acoustic energy. The ear perceives a 10dB difference as a doubling in volume, and IIRC the average person can just barely distinguish a 1dB difference. A car interior at highway speeds is 60-70 dB, a vacuum cleaner in the 80's to 90dB, I think.
Contrary to popular belief, there doesn't seem to be a huge difference between shotgun, pistol, and rifle noise levels, although the sound spectrum is undoubtedly different. There is a correlation with caliber (e.g., .30-06 is louder than .223 and .357 is considerably louder than 9mm or .45), but the most striking difference to me is how much louder a muzzle brake makes a hunting-caliber rifle (nearly 6 dB louder than an unbraked rifle, using an 18" .30-06 as a comparison, or translates to approximately 4 times the radiated acoustic energy). There's a tight correlation with barrel length (shorter is louder for any given caliber), but also less correlation with velocity than I expected (i.e., a slowpoke .30-30 round out of a 20" barrel is a smidgen louder than a faster but much smaller .223 round out of an 18" barrel). For all the 7.62x39mm shooters out there, I'd assume the sound levels would be about the same as .30-30, which it resembles.
Personally, I always double up when shooting (foam earplugs AND slimline earmuffs, I like Silencio plugs and Peltor Shotgunner muffs), but these figures are a good argument for doing that. Consider that typical hearing protection reduces the noise by ~30 dBA, and you're still looking at >130dB for some guns, and >=140dB when shooting a heavy hunting-caliber rifle with a muzzle brake, so with either plugs alone or muffs alone, you could still have sound levels higher than you'd like. I personally wear both.
