Final answer:
A "short barrel" firearm commonly refers to a rifle with a barrel less than 16 inches or a shotgun with one less than 18 inches, and an overall firearm length less than 26 inches, as defined by federal law.
Step-by-step explanation:
According to the Penal Code, a "short barrel" firearm refers to a gun with a barrel length that is shorter than the minimum length prescribed by law. Typically, this means any rifle with a barrel length of less than 16 inches or any shotgun with a barrel length of less than 18 inches, as measured from the front face of the bolt or breech to the end of the barrel. Moreover, the overall length of the firearm must also be considered as firearms must typically be over 26 inches in overall length to not be classified as a short-barreled rifle or shotgun by federal law in the United States. Please note that these definitions can vary by jurisdiction and should be confirmed with the specific state or country's regulations.