United States Supreme Court’s Moncrieffe holding applies retroactively to require reversal of illegal reentry conviction because California firearm offense was not a categorical match to federal aggravated felony firearm offense. Defendant, a Mexican citizen who became a lawful permanent resident in 2000, was convicted of illegal gun possession under former Penal Code section 12021, subdivision (c)(1) (now Pen. Code, § 29805). In 2005, he was removed to Mexico because his conviction constituted an aggravated felony firearms offense (8 U.S.C. § 1227(a)(2)(C)). He was convicted of illegal reentry after he attempted return to the U.S. On appeal, he claimed his removal order was invalid because his California conviction was not a categorical match for the federal firearm aggravated felony. Held: Reversed. After Moncrieffe v. Holder (2013) 133 S.Ct. 1678, a conviction under a state firearm statute lacking an exception for antique guns is not a categorical match for the federal firearms ground for removal. A core feature of the categorical approach focuses on the minimal conduct that would establish the statutory definition of the offense. If the least culpable conduct would result in conviction under the state statute, but trigger only misdemeanor punishment under federal law, the conviction does not necessarily correspond to an offense that is punishable as an aggravated felony. To defeat the categorical comparison, there must be a realistic probability the state would apply its statute to conduct that falls outside the generic definition. Here, the federal definition of a “firearm” specifically exempts antique firearms while the California definition of a “firearm” does not. California does prosecute cases involving antique firearms under former section 12021, subdivision (c)(1). Thus, section 12021, subdivision (c)(1) and the federal firearm aggravated felony are not a categorical match and defendant’s conviction did not mandate removal.