While interesting information I doubt the decibel meter is capable of measuring very short duration impulse noise accurately.
The numbers themselves tell you this is most likely true. 40dB is equivalent to a whisper, 60dB is a normal conversation. A suppressed rifle shot is not going to be this quiet.
I'm sure the suppressor nevertheless is very effective. What I can't understand about Gunworks suppressors is they always look monstrous in size compared to the competition.
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