Originally Posted By: carp

My understanding from WiKI;

Auto fire - you press the trigger to fire and press again to stop.
No, you press the trigger to fire and when you release it stops firing. I don't know of any firearm that requires you to press the trigger again to stop firing. That would be insane, what if you dropped the weapon while it was firing? It would continue to fire.
semi auto - you press the trigger and you have to hold the trigger to continue to fire.
That's not the way a semi works, from wiki;

A semi-automatic, or self-loading, firearm is a weapon that performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine. Typically, this includes extracting and ejecting the spent cartridge case from the weapon's firing chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber. Although automatic weapons and selective fire firearms do the same tasks, semi-automatic firearms do not automatically fire an additional round until the trigger is released and re-pressed by the person firing the weapon.

Unlike semi-automatic firearms, which require one trigger pull per round fired, a machine gun is designed to fire as long as the trigger is held down.