>>With a pump, you'll hear the second round being chambered (no silencer for the pump action). You can tell a pump w/o seeing it.
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>>Semi autos don't have that second (open) pipe sticking all the way out.
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>Are you willing to bet your life in your knowledge? First you said it was visually easy to pick them up, now you said it is for the sound you can pick them up, what is next, the smell?

If I was intent on invading someone's home and they did the Biden maneuver, yes I'd bet my life on my knowledge. I'm more than a little familiar with firearms.

The sound is an additional giveaway on the type of weapon. Obviously you are at the level of Jean on weapons knowledge.