To be fair, the criteria are very precise, they’re just only vaguely related to reality.
My favorite is the double-barreled 1911 pistol. It has two triggers, because if it only had one trigger it would be a machine gun (it would fire multiple bullets with one pull of the trigger). But physically it would never work if it didn’t always fire both barrels at exactly the same time, so it only has one slide and both hammers are connected to each other. But because you have to drop two sears with two triggers before it will fire apparently it’s totally legal.
Depending on atmospheric conditions bullet contrails can be very visible. Looks real to me.