Which of the following weapons is fired from an open bolt?

The M249 SAW (Squad Automatic Weapon) is indeed fired from an open bolt. The open bolt operation is a mechanism that allows the bolt to remain to the rear when the weapon is not firing, which means that the chamber is not closed until the trigger is pulled. This design is particularly effective for machine guns as it helps to manage heat build-up during sustained fire and reduces the chance of a cook-off, where a round can accidentally fire due to heat in the chamber.

In the case of the M249 SAW, this design allows for a more reliable cycling of ammunition and aids in cooling, which is crucial during extended fire scenarios. The other weapons mentioned, such as the M4, .50 cal, and MK19, typically operate using closed bolt designs, where the bolt is forward and the chamber is closed when the weapon is not firing. This design improves accuracy and is more suited for precision shooting, making it less favorable for the high rates of fire typically associated with machine guns like the M249.