A federal court holds that someone being prosecuted for possessing a gun after having been convicted of a domestic violence misdemeanor is constitutionally entitled to present an affirmative defense "that he posed no prospective risk of violence" (which I take it must mean no prospective risk of violence beyond that posed by the average person). The jury would thus be instructed that, if it agrees with the defendant that he posed no prospective risk of violence, it should acquit despite the flat prohibition imposed by the statute.

Here is the meat of the opinion, U.S. v. Engstrum (Stewart, D.J.) (June 15, 2009):

This matter is before the Court on Defendant's Motion for Jury Instruction regarding his possession of a firearm. The Court previously denied Defendant's Motion to Dismiss Indictment, in which Defendant argued that the Second Amendment to the U.S. Constitution protected his right to have a firearm in his house for home and self defense. In its April 17, 2009 Order, the Court found that strict scrutiny was required to justify a deprivation of an individual's Second Amendment right to keep and bear arms. The Court also found that 18 U.S.C. § 922(g)(9), which prohibits the possession of firearms by those previously convicted of a domestic violence misdemeanor, passed strict scrutiny. Finally, the Court found that § 922(g)(9) was, therefore, presumptively lawful, but that the presumption could be rebutted by a showing that the individual charged under § 922(g)(9) posed no prospective risk of violence. With regard to the Defendant, the Court found that it could not say, as a matter of law, that the Defendant posed no prospective risk of violence.

Defendant concedes that he is a restricted person, otherwise covered by § 922(g)(9). In May 2008, Defendant and his girlfriend (the "Girlfriend") were residing at a home in West Valley City, Utah (the "Residence"). On May 9, 2008, Defendant and the Girlfriend got into an argument and the Girlfriend left the Residence. On May 10, 2008, the Girlfriend returned with the police to retrieve her personal belongings, accompanied by a friend, who waited outside the Residence while the Girlfriend entered to retrieve her belongings. Defendant refused to return her things, and an argument ensued. During that argument, Defendant grabbed the Girlfriend's arm, and the Girlfriend claims she feared for her safety. The Girlfriend attempted to use pepper spray on the Defendant, but the canister did not work. Defendant took the pepper spray away from the Girlfriend and was successful in using it on her. The Girlfriend then left the Residence and called the police.

When police arrived at the Residence, the Girlfriend informed them that Defendant kept a gun in his bedroom, although the gun was never used or displayed in any way by the Defendant prior to the police arriving. Defendant allowed the police to enter the Residence, where one officer noticed an unspent round on the floor of the Residence. When officers inquired about the gun, Defendant advised them that it was in his bedroom dresser drawer, and that he had unloaded it when he learned that law enforcement would be arriving at the Residence. The officers found the unloaded gun from the bedroom dresser drawer. The gun was not taken from the Residence at that time.

On May 22, 2008, West Valley Police contacted Defendant and inquired about the gun. Defendant indicated that he owned the gun and that it was a gift from his father. There is no evidence to indicate that Defendant had ever used the firearm. However, Defendant was advised that he could not have a gun due to a prior misdemeanor domestic violence conviction. Defendant indicated to police that he would surrender the gun and ammunition. Police arrived at the Residence later that day and Defendant signed a consent to search form and surrendered the gun and ammunition....

The Court finds that Defendant may raise, as an affirmative defense, that the charged offense may not be applied to him because he posed no prospective risk of violence. Such a defense is in keeping with the law stated in the Court's April 17, 2009 Order. The Court also finds that the affirmative defense raised by Defendant does not negate any element of the offense charged. Therefore, while the government must prove every element of the charged offense beyond a reasonable doubt, if Defendant chooses to argue that he posed no prospective risk of violence, Defendant will bear the burden of proving his defense to the jury by a preponderance of the evidence. However, the defense must be supported by sufficient evidence. Therefore, the Court will only instruct the jury on Defendant's defense if the Court finds that, during the course of trial, Defendant has presented sufficient evidence to convince a reasonable jury that he does not pose a prospective risk of violence. In the event that Defendant meets that burden, the Court will instruct the jury regarding Defendant's proposed Second Amendment defense in the following terms:

The Second Amendment to the United States Constitution guarantees the fundamental right of individuals to keep and bear arms. That right may only be infringed when the restriction is narrowly tailored to meet a compelling government interest. You are instructed that 18 U.S.C. § 922(g)(9), the crime for which Defendant is charged, is, as a matter of law, a lawful and constitutional restriction of the Second Amendment rights of those who pose a prospective, or future, risk of violence.

If you find that the government has proved beyond a reasonable doubt the elements of the charge against him, as set forth in Jury Instruction Number ____, regarding Count I, you are instructed that Defendant is presumed to pose a prospective risk of violence. However, Defendant is entitled to offer evidence to rebut that presumption and show that he did not pose a prospective risk of violence. It is the burden of the Defendant to prove to you, by a preponderance of the evidence, that he did not pose a prospective risk of violence.

Therefore, if you find that the Defendant did not pose a prospective risk of violence, he may not be deprived of his Second Amendment rights, and you must find him not guilty. However, if you find that the government has proved beyond a reasonable doubt the elements of the charge against him, and that the Defendant has not proved, by a preponderance of the evidence, that he posed no prospective risk of violence, you must find the Defendant guilty.

By the way, all I could find from Pacer about Engstrum's past domestic violence misdemeanor conviction was that it was a "domestic violence assault" that had happened in "Midvale Justice Court in 2007." (The statement of facts above describes only the conduct that led to the discovery of the gun, but it was the 2007 conviction that caused Engstrum to be prosecuted for "possession of firearms by those previously convicted of a domestic violence misdemeanor.") Presumably the jury would be told about the circumstances of this past conviction, as well as about other things, in determining whether Engstrum indeed "pose[d] a prospective risk of violence" at the time of possessing the gun.