Joey Fishkin probably has said all that needs to be said about the various justices' attempts to do constitutional theory in Rahimi. He may be too subtle, though. So here's the hit-you-on-the-head-with-a-hammer version.
Right-wing op ed writers who think they know something about the Constitution (I'm looking at you, George Will, but there are others) are probably already in print, and certainly will be soon enough, with pieces describing the conservative justices' separate opinions as "brilliant," "careful," "important," and other praise words. Acolytes in the legal academy will follow with similar praise perhaps a bit toned down.
Were the opinions to be submitted as essays in a decent law school course or seminar in constitutional theory, they'd get something close to the median grade (these days, I suppose, a B+). They demonstrate a reasonable grasp of the fundamental issues, though they overlook some subtleties in the best of the works they refer to. They indicate some awareness of the pressure points on their theories but at best offer promissory notes about what future theoretical development might yield. They demonstrate no awareness whatever of the feebleness of what they do offer as moving the ball forward, with Justice Barrett's Goldilocks line being the best evidence of that.
It's not worth going through the opinions in detail with the kinds of comments I'd make on seminar papers; others will surely do that. Here I simply raise a couple of questions. Given that the proper thing to do is extract a guiding principle from the historical and traditional materials, how do you determine the level of generality at which to state that principle? Justice Scalia offered one answer in Michael M. (the most concrete, least abstract level), but it didn't stick, for good reason (it might work for a handful of problems but couldn't work as a general guide). Roberts's opinion says that "responsible" isn't the right level because its too vague--but that's not right either: the term has a reasonably clear but rather large core meaning, with vagueness at the edges, it's just that the core meaning sweeps more broadly than Roberts would like. That suggests another candidate for determining the level of generality of the principle: choose the level that, with respect to the problem at hand (because the level is likely to vary depending upon the problem), yields the best social policy result. But, of course, the opinions can't say that because the point of the exercise is to prevent judges from relying upon their policy preferences when they decide cases.
Or, maybe not. Maybe you can't rely directly on your policy preferences to justify a decision but you can do so indirectly, through the choice of the level of generality. They can't say that, though, and sustain their criticism of their liberal adversaries, because those adversaries never relied on policy preferences directly either. Maybe "never" is too strong, but one or two aberrations wouldn't weaken the point, and the most commonly offered example fails. The liberals in Griswold didn't strike the statute down because it was uncommonly silly; they offered an account of a constitutional right to privacy in intimate matters that they found rooted in constitutional text and tradition. Justice Breyer's typical use of policy arguments, to uphold statutes against constitutional challenges, is Thayerism: "I have no views on the policy issues here but here's a lot of evidence to support the proposition that it's reasonable for a legislature to conclude that this is good policy."
The second question is related: How do you escape the suspicion that you're picking the level of generality to produce results that fit with your sense of good policy outcomes? Understandably, no one wrote an opinion in the vein of Kennedy's in the flag-burning case: "I hate the fact that Rahimi isn't going to be allowed to own a gun, but my constitutional theory dictates that Roberts's principle is at the right level." Indeed, in Rahimi it would be really hard to write such an opinion because Roberts's principle is that you can disarm (temporarily) people who are found by a court to pose a credible threat to the physical safety of others--and it's really hard to disagree with that as a matter of policy. (Or is the suggestion that a ban wouldn't be justified if the determination were made by an administrator rather than a judge [after similarly truncated procedures], or if the threat were to the psychological safety of another, or if the threat were to a class of people rather than "another"? I'd like to see the explanation for those possible limitations on the principle's scope.)
Richard Posner once concluded that it was laughable to think that the justices were the nine best lawyers in the country, or were among the best one hundred. He thought that ten thousand might be the right number. Or, to return to the beginning, B+.