Mo. Rev. Stat. § 571.070: Possession of firearm unlawful for certain persons — penalty — exception.

1. A person commits the offense of unlawful possession of a firearm if such person knowingly has any firearm in his or her possession and:

(1) Such person has been convicted of a felony under the laws of this state, or of a crime under the laws of any state or of the United States which, if committed within this state, would be a felony; or

(2) Such person is a fugitive from justice, is habitually in an intoxicated or drugged condition, or is currently adjudged mentally incompetent.

2. Unlawful possession of a firearm is a class C felony, unless a person has been convicted of a dangerous felony as defined in section 556.061, or the person has a prior conviction for unlawful possession of a firearm in which case it is a class B felony.

3. The provisions of subdivision (1) of subsection 1 of this section shall not apply to the possession of an antique firearm.