What is the main differentiating factor between a submachine gun and a machine gun?

The main differentiating factor between a submachine gun and a machine gun is that submachine guns fire pistol rounds, while machine guns typically fire rifle rounds and are designed to be belt-fed. This distinction is rooted in the intended use and operational context of each type of firearm.

Submachine guns are generally lighter and more compact, making them ideal for close-quarters combat. Their ability to fire pistol cartridges allows for improved controllability and a higher rate of fire in confined environments. On the other hand, machine guns are built to deliver sustained firepower with larger-caliber ammunition, typically from belts or linked ammunition, which provides the capacity to engage targets over longer distances with more stopping power.

In summary, the combination of caliber, feeding mechanism, and intended operational use clearly distinguishes submachine guns from machine guns, with submachine guns focusing on mobility and quick handling, while machine guns emphasize sustained fire and power.