I agree that we should certainly be fighting this, just not with bullets. And I agree that there's a serious problem when an unaccountable, unelected agency can make up the rules as it goes along. Believe it or not, that's also being fought: the seminal case giving administrative agencies that power is Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984), which held that
First, always, is the question whether Congress has directly spoken to the precise question at issue. If the intent of Congress is clear, that is the end of the matter; for the court, as well as the agency, must give effect to the unambiguously expressed intent of Congress. If, however, the court determines Congress has not directly addressed the precise question at issue, the court does not simply impose its own construction on the statute . . . Rather, if the statute is silent or ambiguous with respect to the specific issue, the question for the court is whether the agency's answer is based on a permissible construction of the statute.
As it turns out, Congress has written a definition of machine gun, and it defines it as more than one round fired by a single operation of the trigger. There's room to challenge such an ATF decision in court even under so-called "Chevron deference." Moreover, the Court has opined that it may be time to revisit Chevron and rein it in. Such a case is not presently before the Court, but it's definitely coming; the Court has noticed that there's an awful lot of power in unaccountable hands, and is not impressed.
As to a Fifth Amendment violation...that was settled long ago, unfortunately. The Fifth Amendment states, in part, "...nor shall private property be taken for public use, without just compensation [emphasis mine].” Note the italics: the property in question isn't being taken for public use (such as a road, military base, or--in the bass-ackward case of Kelo--redevelopment), but being declared contraband. I can't think of the case offhand, but that's been precedent for a long time, and it does make a certain amount of sense in the context of the amendment as written. I don't like it, but there it is.
And before anybody thinks of the ex post facto clause, nope, it's not that either. That's making something illegal retroactively; this is saying "it's legal up to x date; after that, you're busted."
So...definitely keep your powder dry, but don't be too quick to start dropping hammers.
The issue is that congress has created these unelected agency's to regulate for them so they can be re-elected without having to actually do something that might offend someone.
It's the EPA that did that!
It's the ATF that did that!
It's the WTF ever letter organization you may choose that did that!
The Republicans voted how many times to repeal obamacare when a minority, and when becoming a majority, became feckless and timid, refusing to vote it out for fear of repercussions by voters?
They are treating the 2A the same, allowing the alphabet agencies to do their dirty work instead of standing up for the people that elected them.
100% of the fault lies within the halls of congress.