Which of the following is not a type of ammunition used in the M9 pistol?

The M821 ball is not a type of ammunition used in the M9 pistol, making it the correct choice in this context. The M9 pistol, a widely used sidearm in military applications, typically employs specific types of ammunition designed for its operational needs.

M881 ball is a standard 9mm NATO round that is compatible with the M9, making it a common choice for training and operational use. The M862 plastic is designed for training purposes, providing a non-lethal option for practice. M118 special, while primarily known as a long-range rifle cartridge, is not utilized in the M9, reinforcing the understanding that different firearms require specific ammunition types.

The M821 ball round, however, does not align with the specifications necessary for firing from an M9 pistol, thus clarifying why it is not associated with that particular firearm. Understanding the appropriate ammunition types for specific weapons ensures safety and effectiveness during training and operational engagement.