Originally Posted by cptsplashdown
...There is no probable cause to believe that a crime has been committed....
Actually, I suspect that a judge would say that there is probable cause. Fully automatic firearms, short barrel rifles, suppressors, etc., are, in fact, illegal, UNLESS they have been properly registered under the NFA. Therefore it could easily be argued, that an LEO seeing you with an NFA firearm has reason to believe that you are in possession of contraband if you can't show that you have complied with the requirements of the NFA.
"Probable cause" is "sufficient reason based upon known facts to believe a crime has been committed or that certain property is connected with a crime...." Possession of an NFA weapon or device without having satisfied the NFA formalities is a crime. An LEO seeing an individual with an NFA weapon or device has no way of knowing if the necessary formalities have been satisfied (unless the person in possession can produce the appropriate paperwork) and therefore has reason to believe a crime has been committed. (And no, he doesn't have to take your word for it.)