The twang of freedom.
People get real hung up on this but it's no different to a shotgun with the recoil spring in the but. My Beretta 3901 used to go "twang-kerclunk" every time I fired it.
Having said that an adjustable gas block seemed to tame the noise in my rifle and it's not noticable now.
Commenting on the twang is the first thing people do after shooting a real AR. They didn't add the twang into counterstrike so people don't expect it,.
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