The notorious "Sporting Purposes" clause is rather ambiguous, by design. It gives the ATF an exceptional amount of arbitrary power to declare something to be "sporting" or "unsuitable for sporting purposes" simply by decree. Not so surprisingly, their concept of "sporting" is pretty much restricted to expensive bolt-action deer rifles, aristocratic clay pigeon shotguns, and MAYBE dangerous game double rifles.
The very notion that competition shooting even exists with semi-automatic rifles, handguns, or tactical shotguns is earnestly ignored by the agency. That gives them just enough excuse to (mostly) publicly get away with declaring anything potentially remotely useful for... ahem... social purposes to be "unsporting".
We "inherited" the entire notion of "sporting purposes" justification from - believe it or not - the original Nazi gun control laws. There is congressional record of the author of the 1968 Gun Control Act requesting an english translation of those laws from, if memory serves, either the Library of Congress or the Congressional Research Service. Significant portions of the more restrictive parts of the 1968 GCA are, essentially, cribbed wholesale from Nazi laws, and after nearly 60 years, much of the public has now been acclimated to consider living under Nazi firearm regulations as "reasonable". I wish I was joking.
Understanding why ATF is so inherently hostile to private gun ownership in the first place is a very long, convoluted and unhappy story wrought with corruption, authoritarianism, bureaucratic myopia and a long-accumulating phenomenon of an entire government agency worth of people who continually have chosen to find some way to justify the existence and importance of their jobs and salaries, even when doing what is ethical, constitutional, or simply legal demands otherwise. With several decades of infusion at the upper politically-appointed levels of authoritarian, statist, anti-gun idealogues, you have a quick recipe for bad things and gross abuses of power. ...such as their current activities, which have consisted mainly of illegally supplying thousands of weapons to international narco-terrorist cartels, then parading fallacious or wholly fabricated statistics around the media to justify even more draconian abrogations of American rights.
I can think of nothing that would scare the ATF more than 3D printing technology that can literally print a modern firearm becoming affordable and widespread, because it would make them almost entirely irrelevant. They would fade away in obsolete obscurity much like a federal bureau that existed to tightly control horse buggy design and possession, faced with the announcement by Henry Ford of the automobile for the common man.