automatic

An automatic firearm is a firearm that continuously chambers and fires rounds when the trigger mechanism is actuated. The action of an automatic firearm is capable of harvesting the excess energy released from a previous discharge to feed a new ammunition round into the chamber, and then ignite the propellant and discharge the projectile (either bullet, shots or slug) by delivering a hammer/striker impact on the primer.
If both the feeding and ignition procedures are automatically cycled, the weapon will be considered "fully automatic" and will fire continuously as long as the trigger is kept depressed and the ammunition feeding (either from a magazine or a belt) remains available. In contrast, a firearm is considered "semi-automatic" if it only automatically cycles to chamber new rounds (i.e. self-loading) but does not automatically fire off the shot unless the user manually resets (usually by releasing) and re-actuates the trigger, so only one round gets discharged with each individual trigger-pull. A burst-fire firearm is an "in-between" of fully and semi-automatic firearms, firing a brief continuous "burst" of multiple rounds with each trigger-pull, but then will require a manual re-actuation of the trigger to fire another burst.
Although all semi-automatic, burst-fire and fully automatic firearms are "automatic" in the technical sense, the terms "automatic weapon" and "automatic firearm" are used to describe fully automatic firearms. Use of more specific terms such as "full-auto","semi-auto" or "burst" can help avoid confusion. Automatic weapons that can switch between the aforementioned firing modes are known as selective fire weapons.
Automatic firearms are further defined by the type of cycling principles used, such as recoil operation, blowback, blow-forward or gas operation.

View More On Wikipedia.org