One of the primary criticisms of a four-officer diamond formation in a hallway is the increased potential for crossfire and unsafe muzzle orientation, particularly in the dynamic, close-quarters environments that patrol officer's encounter.

Here are the main concerns:

• Crossfire risk: Officers are positioned on multiple sides of the formation. If a threat suddenly appears from a doorway, side room, or intersecting hallway, officers may have teammates directly or partially in their line of fire. 
• Muzzle discipline: In the compressed space of a hallway, weapons may momentarily cover fellow      officers as the formation pivots or reacts. 
• Limited maneuverability:A diamond takes up much of the hallway width. Officers can interfere with one another when trying to move to cover or react to a rapidly changing threat. 
• Slower reaction: Maintaining formation requires coordination. During an active shooter response, speed and decisiveness are often more important than preserving a geometric formation. 
• Reduced fields of fire: The rear and side officers may have limited ability to engage without shooting across or near teammates. 

By contrast, many contemporary patrol training programs emphasize:

• A simple linear or offset file. 
• Officers responsible for their assigned sectors. 
• Dynamic movement based on the environment rather than maintaining a fixed formation. 
• Using available cover and angles. 
• Maintaining spacing that minimizes crossfire while maximizing observation. 

This reflects the broader evolution in active shooter tactics after incidents such as Columbine High School massacre and subsequent reviews of law enforcement responses. Many instructors now teach that patrol officers should move quickly toward the threat while maintaining 360-degree awareness, rather than focusing on preserving a rigid formation.

That said, there is no single nationwide rule that prohibits the diamond. Some agencies and specialized teams may still teach or use variations of it for specific missions, such as deliberate searches or certain tactical operations. The appropriateness of any formation depends on the mission, staffing, and agency doctrine.

For a patrol response to an active shooter in a hallway, the trend over the last decade has generally been toward simpler movement techniques that reduce crossfire hazards, improve speed, and allow officers to adapt rapidly to changing conditions. Given your work in patrol-centric active shooter training, this point aligns with emphasizing that formations should support the mission rather than become the mission.