A cheap noise meter and microphone do not do a good job of accurately determining levels from short duration noises like gun shots. A meter/microphone combo needs to have no more than a 20 micro-second response time according to the experts. Otherwise you end up getting low levels like 80 to 100 decibels when is it obviously louder than that. This kind of setup is about $4000 and up.
http://www.elcaudio.com/decibel.htm
http://www.silencertests.com/results.htm
http://silencertalk.com/2209-method.htm
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