What FM is designated for 40mm grenade launchers?

The correct designation for the field manual associated with 40mm grenade launchers is FM 3-22.31. This manual specifically covers the employment, tactics, techniques, and procedures for operating and maintaining the M203 and other related grenade launchers. It is crucial for soldiers to be familiar with this manual, as it provides comprehensive guidance on safety measures, operational capabilities, and performance standards for these weapons systems.

The other manuals listed deal with different topics within military training and operational guidelines. For instance, FM 3-23.31 covers the training and use of the M320 grenade launcher system, which is separate from the 40mm grenade launchers indicated in the question. Understanding the specific field manuals helps ensure that personnel are well-prepared and knowledgeable about their equipment, which is critical for successful operations.