Oppo, I don't know how to make it any clearer to you. You're citing a bunch of examples about RS that have no background info. Would a reasonable and prudent person believe that a person drinking a beverage out of a cup is drinking alcohol? Not necessarily. Would a reasonable and prudent person believe that a person drinking a beverage out of a cup, stumbling around, slurring their words is drinking alcohol? Yes, that would be reasonable. Police officers have to take into account the totality of the circumstances they are faced with, which is never a simple black and white proposition.
Those two were walking down the street with a weapon that looked like a suppressed MP5. There was reasonable suspicion there for that contact. What you're failing to realize is that the kind of gun itself doesn't matter; it only mattered in that case to give the officer probable cause to remove the weapon from his person to inspect it. Even if they were walking down the road carrying a Ruger 10/22, a simple contact would STILL be justified to ask them their intentions, determine their mindset, and to find out if anything fishy was going on. If they say "oh we're just exercising our 2A rights walking down to the range", hey that's cool, have a good day. However, if they say "Well...Sir MonkeyFist the Demon God has commanded me to walk down to this street corner and kill all non-believers", well, then we have a problem! It's called investigating, man. That's how we catch bad guys - it doesn't mean we're trampling on peoples rights for asking questions. If you don't believe there was at least reasonable suspicion there, then unfortunately your perception of how law enforcement is supposed to function is a little clouded, as is your knowledge of the law.
If you think that a citizen calling in doesn't matter, let me ask you this: how else do you think cops receive about 90% of the calls they go on? Just because a concerned citizen calls something in doesn't mean that enforcement action will be taken - it's simply the first step in allowing the police to investigate (there's that word again!) whether or not a crime has been committed. We can't just ignore calls that come in because it might not be criminal activity; it might be or it might not be. Take DUI reports for example. Are we not going to investigate one just because the person may not be drunk? That's why we investigate, and if all is well, then no enforcement action is taken. Just like what happened with that contact.
Now, if the officer had acted differently or that contact ended in an arrest or something else inappropriate for the situation, then my opinion would be different regarding the outcome - but not the contact.
Regarding tax stamps, you need to read up on that as well. They aren't just for SBR's and supressors. They're for legally transferred machine guns too, among other things. My point in bringing that up is that while they SAID that the guns weren't fully automatic, guess what? People lie! All the time! My point was that they didn't offer one up, so if the gun did in fact turn out to be an MP5, it would be more likely that they were in possession of an illegal machine gun rather than one legally purchased.