Which weapon is NOT mentioned as a potential for warning shots?

The M1911 is a type of semi-automatic pistol that is traditionally not referenced as a weapon for firing warning shots in military or tactical contexts. Typically, warning shots are fired using firearms that are either long-range or have higher caliber projectiles designed to be more effective in a warning capacity, such as rifles and machine guns. The other weapons listed – the M4/M16 rifles, the M240 machine gun, and the .50 caliber crew-served weapons – are designed for a more significant impact and control over engagement scenarios, making them more suitable for the purpose of warning shots. Thus, the M1911 does not align with the standard expectations for warning shot applications, which is why it stands out as the correct answer in this context.