I composed this as a response to a commenter on a Volokh post, here. But it's too long and too far off the main subject of the post to justify putting it up in Eugene's comments section, so I'll stick it here instead -- slightly edited, of course.
The issue of federal prohibited persons -- by which I mean people prohibited by federal law from possessing firearms -- is a lot more complicated than most people realize. The jurpisprudence in this area is an utter mess, and the federal-state interplay in many cases is a killer.
The main federal statute at issue is 18 U.S.C. section 922(g), which reads:
(g) It shall be unlawful for any person—
(1) who has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year;
(2) who is a fugitive from justice;
(3) who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802));
(4) who has been adjudicated as a mental defective or who has been committed to a mental institution;
(5) who, being an alien—
(A) is illegally or unlawfully in the United States; or
(B) except as provided in subsection (y)(2), has been admitted to the United States under a nonimmigrant visa (as that term is defined in section 101(a)(26) of the Immigration and Nationality Act (8 U.S.C. 1101 (a)(26)));
(6) who has been discharged from the Armed Forces under dishonorable conditions;
(7) who, having been a citizen of the United States, has renounced his citizenship;
(8) who is subject to a court order that—
(A) was issued after a hearing of which such person received actual notice, and at which such person had an opportunity to participate;
(B) restrains such person from harassing, stalking, or threatening an intimate partner of such person or child of such intimate partner or person, or engaging in other conduct that would place an intimate partner in reasonable fear of bodily injury to the partner or child; and
(C) (i) includes a finding that such person represents a credible threat to the physical safety of such intimate partner or child; or
(ii) by its terms explicitly prohibits the use, attempted use, or threatened use of physical force against such intimate partner or child that would reasonably be expected to cause bodily injury; or
(9) who has been convicted in any court of a misdemeanor crime of domestic violence,
to ship or transport in interstate or foreign commerce, or possess in or affecting commerce, any firearm or ammunition; or to receive any firearm or ammunition which has been shipped or transported in interstate or foreign commerce.
Two important definitions for purposes of interpreting these provisions can be found at 18 USC 921(a)(20) and (33).
Some of these prohibitions obviously are temporary -- e.g., those relating to fugitives from justice and persons subject to certain restraining orders. The prohibition in those cases last only as long as the predicate condition continues. But what about the prohibitions that depend upon facts -- such as conviction of a crime or adjudication as a mental defective -- that, once established, can never really be "un-established." Can people subject to those prohibitions regain the right to possess firearms? Well, it depends.
State-convicted felons* can regain the right to possess firearms under federal law through a pardon, expungement, set-aside or restoration of civil rights by the state of conviction. Pardons, expungements and set-asides are quite rare and difficult to obtain (depending on the state, of course), which leaves restorations of civl rights as the only practical method for most state felons to regain the right. Generally speaking, restoration of civil rights means the restoration of the rights to vote, serve on a jury, and hold public office -- combined with the requirement that state law explicitly prohibit the felong in question from possessing a firearm. In the view of the United States Supreme Court, that means that state law must not treat the felon at all differently from a non-felon with respect to the right to possess firearms. Caron v. United States, 524 U.S. 308 (1998).
At least two problems arise from this interpretation. First, what happens if state law never stripped the felon of his civil rights to begin with? At least five federal courts of appeal have held that you can't restore what's never taken away; in other words, convicted felons in those circuits simply cannot have their civil rights "restored," and thus are effectively barred from ever possessing firearms. Only one circuit that I know of -- the First Circuit -- has held that never having stripped a felon of his civil rights is tantamount to a restoration of rights. Second, it turns out that a great many states do in fact have statutes that treat felons differently than non-felons for purposes of possessing firearms, even if only slightly so. For example, at issue in Caron was a Massachusetts law that permitted convicted felons to possess rifles and shotguns, but not handguns. The Supreme Court concluded that Caron's right to possess firearms had not been fully restored by state law, and that it was therefore unlawful (under 18 USC 922(g)(1)) for him to possess any firearm -- even firearms that Massachusetts law permitted him to possess. Many states, like Massachusetts, try to take a nuanced approach to these matters; what they don't seem to understand is that 922(g)(1) and Caron don't leave any room for nuance. They demand an all-or-nothing approach.
The bottom line is this: it's a rare state felon who can realistically expect to ever regain the right to possess a firearm. (There are some exceptions. Idaho restores felons' civil rights by operation of law something like five years after completion of sentence. Don't quote me on that, though; it could be ten years.)
Even so, state felons are in far better shape than federal felons. As things currently stand, federal felons -- i.e., persons convicted in federal court of a felony -- are essentially completely out of luck. The restoration of rights provision in 18 USC 925(c) has been defunded in federal appropriations bills for the past fifteen years, and there's no reason to believe that will change in the foreseeable future. In United States v. Bean, 537 U.S. 71 (2002) the Supreme Court held that BATF's failure to act on an application for relief from disability (because it was prohibited from spending funds to do so) was did not constitute a denial of the application; thus, judicial review was unavailable. What this means is that you can apply to BATFE for a restoration of the right to possess a firearm, but BATFE can't act on your request -- and there's nothing you can do about it.
Expungements and set-asides are next to impossible to obtain at the federal level, particularly in cases that don't involve actual innocence. [Someone wittier than I can insert an appropriate joke here regarding cronyism, Whitewater, the Rose Law Firm, Clinton, Ford and Nixon or what-have-you.] That leaves presidential pardons. Good luck getting one of those. In short, if you're a federal felon you are, to put it very bluntly, shit out of luck when it comes to ever again lawfully possessing a firearm.
Convictions for misdemeanor crimes of domestic violence also, in most cases, effectively work lifetime prohibitions under current law. The problem is that few if any states** strip misdemeanants of their civil rights for misdemeanors, and thus there can be no "restoration," and the applicable federal statute explicitly requires a deprivation of civil rights before they can be considered to have been "restored." (Expungements, set-asides and pardons are, again, not realistic alternatives for the vast majority of misdemeanants. In some states there's no such thing as a pardon for a misdemeanor!) Only one circuit that I know of has held otherwise, United States v. Wegrzyn, 305 F.3d 593 (6th Cir. 2002), and I'll be surprised if any of its sister circuits (with the possible exception of the Fifth) follow suit.
As an aside, the states could easily change this situation but to my knowledge only one, Wyoming, has attempted to do so. Unfortunately, Wyoming went about it by instituting a restoration process with relatively strict requirements, available only to first offenders, presided over by a state trial court judge, that results in what Wyoming has decided to call an "expungement." That seems like quite a sensible approach to me, except that it's not at all clear that the Lautenberg Amendment leaves room for sensible approaches. BATFE claims that the resulting "expungement" isn't an expungement within the meaning of 18 USC 921(a)(33) (definition of "misdemeanor crime of domestic violence"), and BATFE may well be right. Wyoming is currently suing to find out.
In addition to MCDV convictions, it's currently not at all clear that anyone who's been "adjudicated as a mental defective or who has been committed to a mental institution" can ever regain the right to possess firearms. Unlike state-law felonies and MCDVs, this prohibition isn't controlled by state law. The only apparent avenue of relief is a federal restoration of rights. But as I noted earlier, you haven't been able to get a federal restoration of rights since 1992. In Galioto v. Dep't of the Treasury, 602 F. Supp. 682 (D.N.J. 1985), the court held that it was a violation of due process and equal protection to permanently prohibit "mental defectives" from possessing firearms despite the possibility of treatment and recovery, especially in light of the fact that convicted felons were subject to no such blanket, permanent prohibition. The Supreme Court granted cert. in Galioto, but the case became moot when Congress passed remedial legislation in the form of an amendment to 18 USC 925(c). Dep't of the Treasury v. Galioto, 477 U.S. 556 (1986). Of course 925(c) is now defunded. But since it's defunded for both federal felons and and "mental defectives," presumably there's no equal protection or due process problem here. So it appears that "mental defectives" are out of luck, too, at least for the time being.
Persons discharged from the Armed Forces under dishonorable conditions are also prohibited persons, 18 USC 922(g)(6), and the only obvious avenue of relief is, again, the now-defeunded section 925(c). See the pattern here?
And this little summary only scratches the surface of the problem. Each state has its own laws regarding who may possess firearms. Even if you manage to regain the right to possess firearms under federal law (and thus, necessarily, the law of the state in which you were convicted), that's no guarantee that the law of the state in which you currently live will recognize the restoration.
The bottom line is that if you value the right to possess guns (without risking federal prison for doing so), you'd better keep your nose clean.
*The federal statute doesn't actually use the word "felon." It prohibits possession of a firearm by anyone convicted "of a crime punishable by imprisonment for a term exceeding one year." 18 USC 922(g)(1). (Note: It is the maximum theoretical sentence, not the sentence actually awarded, that determines whether you're a prohibited person.) But the term "crime punishable by a term of imprisonment exceeding one year" excludes crimes classified as misdemeanors by the state of conviction, unless the maximum punishment for the crime exceeds two years. 18 USC 921(a)(20). For most practical purposes, this limits the effect of the statute to felons. But there are a few states out there with misdemeanors punishable by terms of imprisonment exceeding two years. For whatever reason, "federal or State offenses pertaining to antitrust violations, unfair trade practices, restraints of trade, or other similar offenses relating to the regulation of business practices" are also excluded from the definition of a crime punishable by imprisonment for a term exceeding one year. So certain white-collar felons can possess guns, although the rationale for the distinction escapes me. (People who conspire to bilk consumers out of millions of dollars are trustworthy enough to own guns but, for example, people who grow marijuana to give to terminal cancer patients aren't? Of the thousands of federal laws on the books -- and the many thousand more federal regulations -- under which a person might be convicted, Congress felt compelled to single out a handful of economic crimes as not sufficiently serious to warrant a deprivation of the right to keep and bear arms? That's always struck me as very weird.)
**The vast majority of MCDV convictions occur at the state level, but I suppose there probably are a small number of federal convictions. Relief from those convictions would have to come via 925(c).