It only applies during the Alternative Season, but it's worth noting.
https://mdc.mo.gov/newsroom/mdc-updates-definition-handguns-used-deer-hunting"The updated regulation clarifies what a handgun is for deer hunting during the alternative-methods portion," Doman explained. "We want hunters to be able to confidently identify what handgun configurations, with or without the addition of braces or other accessories, are legal to use during this portion of the deer hunting season. The essential distinguishing characteristic of a handgun is its ability to be operated with one hand, although a second hand can be used as a brace. Any firearm with a total length of over 26 inches is considered a rifle."
The Commission considered and adjusted the definition based on public input received during a formal comment period in April.
The updated MDC definition of a handgun is, "Any firearm originally designed, made, and intended to fire a projectile (bullet) from one (1) or more barrels when held in one (1) hand, and having a short stock designed to be gripped by one (1) hand at an angle to and extending below the line of the bore(s), with a barrel less than sixteen inches (16") in length, measured from the face of the bolt or standing breech (excluding any muzzle device not permanently attached to the barrel), and an overall length less than twenty-six inches (26") as measured between the muzzle of the barrel and the rearmost portion of the firearm (excluding any pistol brace, muzzle device, or other firearm accessory not permanently attached to the firearm). The use of a pistol brace is specifically authorized, and a second hand may be used for support when firing."