This is the first in a series of posts to help educate everyone on some of the misunderstood aspects of firearms. Today's topic: Automatic vs. Semi-Automatic.
An automatic weapon fires multiple rounds (bullets, projectiles) by pulling and holding down the trigger. Because of the automatic's firing mechanism, projectiles will continue to be fired until the trigger is released, or the magazine feeding the rounds is empty.
A semi-automatic weapon fires one round per pull of the trigger. It doesn't matter if you hold the trigger back, it still fires only one round per pull.
Automatic weapons fall into a category of weapons that are heavily regulated by the National Firearms Act (http://en.wikipedia.org/wiki/National_Firearms_Act) of 1934 and the Gun Control Act of 1968. In essence, a civilian cannot purchase an automatic weapon manufactured after 1987. It's worth your time to read the link I've provided regarding all the rules and regulations. Yes, you can buy an automatic weapon manufactured before 1987, but they're horribly expensive (because they're collector's items), to the tune of $10,000 and up. Add to that the rigorous requirements to own and register one, and it's just not feasible for the average citizen to own, for the most part. Failure to follow the rules will result in Federal prison time.
Semi-automatic weapons are what most people have today. Revolvers have fallen out of vogue for the most part. The average hunting rifle, shotgun, and pistol is a semi-automatic. Magazine capacities vary, based primarily on the size of the rounds (bullets) being used. For example, more 9mm bullets will fit in a magazine than a .45 caliber, because of the size difference. There are several popular calibers for rifles, and the type of semi-auto rifle typically only varies in appearance. For example, the Ruger Ranch Rifle in .223 is functionally identical to the AR-15 rifle, although the AR looks "scarier." Functionally, they are no different.
Questions or comments are welcome. I'll talk about magazines and clips in the next of this series. Hope this helps.