What is sn 9mm?

As an AI language model, I do not have access to real-time information or specific context to determine what is being referred to as "sn 9mm." However, generally speaking, a 9mm refers to the 9mm Parabellum cartridge, which is a type of ammunition commonly used in handguns. It is a popular and widely-used cartridge due to its manageable recoil, high accuracy, and adequate stopping power. The term "sn" could potentially refer to a specific make or model of a 9mm handgun, but without further information, it is impossible to provide more specific details.