[JohnKSa]

In specific response, the NFA weapon was not illegally possessed...

You're (intentionally?) missing the point. It goes back to my self-defense analogy. In that analogy I had not broken the law but the evidence made it appear that I had. Since I chose (legally) not to provide the evidence which would have showed my innocence I should expect to be treated like a murderer--arrested and prosecuted. Any LEO who let me go just because I claimed I was innocent would be terribly negligent.

Possession of an NFA weapon is illegal without the proper paperwork (just as shooting someone is illegal without the proper justification) and so in both cases (without the proper paperwork/justification) the persons in question should expect to be treated as if they have committed a crime.

Many people MAY be breaking the law at the range...

If any of them APPEARED to be breaking the law then I would DEFINITELY expect any LEOs present to act appropriately.

What you are refusing to acknowledge is that this is not about someone who "may be breaking the law", it is about someone who is giving every appearance of breaking the law.

1. You are not committing a crime simply by being in possession of an NFA firearm, so long as it is registered, even if the paperwork is not in your immediate possession.

Even assuming that's true, that doesn't mean you will not/should not be TREATED as if you have committed a crime until you present your defense--i.e. your paperwork.

2. It is not appropriate for anyone other than a BATF or DOJ agent to even ask you to produce that paperwork, as it is beyond the scope of the laws that any other LEOs are chartered to enforce.

Assuming that there is no state law that is appropriate--and in many states there are laws which "back up" NFA laws.

You are certainly not entitled to arrest someone for NOT breaking the law...

You don't get a trial right on the spot. LEOs do not determine guilt nor are they required to determine guilt in order to fine/arrest. They operate based on probable cause. If you provide probable cause (and give them no evidence to prove that you are innocent) then you will be fined/arrested.

You're treating this like police have some magical guilt determination equipment available to them--that they can somehow look at you and tell you're innocent even though you appear to be committing a crime. How, exactly is that supposed to work?

...you are simply not entitled to enforce some laws even if they are being broken.

I don't think that's entirely true. I believe you will find that in many cases it is completely legal and proper for one LE organization to apprehend and hold a person for crimes committed under the jurisdiction of another LE organization. They just can't prosecute them--they can only turn them over (per the proper procedures) to the LE organization with jurisdiction.