You see, there is no such thing as a semi-automatic assault rifle. The terms "semi-automatic" and "assault rifle" define two separate types of firearms and are basically mutually exclusive.
A semi-automatic rifle is a rifle that shoots one round per trigger pull (1 pull = 1 bang). Semi-autos have been around for over 100 years and are common for hunting, recreation, and defense. The infamous AR-15 is prime example of a semi-automatic rifle from the late 1950s/early 1960s.
A full-auto rifle is a rifle that shoots multiple rounds per trigger pull (1 pull = bang, bang, bang, bang). In other words, a full-auto rifle is a machinegun. Machineguns have been strictly controlled by the National Firearms Act of 1934 as Class III firearms.
An assault rifle is defined by historical development and the US State Dept and Dept of Defense as a "lightweight, select-fire rifle firing light to intermediate power rounds". The term "select-fire" means the rifle can fire in either the semi-auto or full-auto modes by means of an external selector (hence the name), usually a lever. Since an assault rifle can fire full-auto with the flick of a switch, they are classified as full-auto firearms and therefore fall under the 1934 NFA Class III firearm restrictions. Assault rifles trace their original type to the SturmGewehr 44 of NAZI Germany. These lightweight rifles were less powerful than the main battle rifles most troops carried, were select-fire, and were designed for special uses. A perfect example of an assault rifle is the M-16--it is select fire, is lightweight, and fires one of the lightest, weakest rifle rounds in existance.
A similar term to assault rifle is "assault weapon". This term is disceptive, vague, and redundant. The term has been used for some time to describe everything from revolvers and bolt action rifles to machineguns, but it was the leaders of the gun control movement, Josh Sugarmann and Pete Shields, who deliberately began using the term in order to confuse the public. Memos show that they intended to capitalize on the similarity of the terms and public ignorance in order to scare the public into banning firearms.
The term "assault weapon" never had a clear definition until 1994 when the assault weapon ban defined them. Even then the definitions were problematic and had to be revised. In regards to rifles, the original plan was to include any semi-auto with a detachable magazine, but the gun controllers soon found out that would ban so many firearms that the public outcry would be far too great. The current definition of a rifle classed as an assault weapon is a semi-auto with a detachable magazine and two or more of the following: Pistol grip, Bayonet lug, Flash Suppressor (this is not a silencer--it usually makes the firearm louder), grenade launcher, threaded barrel (for attaching silencers), or collapsable stock. Of all the features, probably the only one that has any sense to it at all is the collapsable stock. When was the last time you heard about a bayonetting? What about an attack with grenade launchers? Flash suppressors simply reduce the amount of 'flame' that comes out of the barrel of the firearm and pose no threat. Silencers can be attached without threaded barrels and again, when was the last time you heard about one being used in a crime? Probably never. Attaching a silencer to a firearm without government permission was already a felony worth 10 years in jail. The fact is that rifle "assault weapons" were used in less than 1% of all firearm related crimes, but the scare tactics of the gun controllers worked and the ban was passed.
The term is redundant because to assault is to attack and a weapon is an object used to attack with, so an assault weapon is literally an "attack object used for attacking".
Some examples of the firearms types listed above would be:
* Pre-ban AR-15 = "assault weapon" or "semi-auto rifle"
* Post-ban AR-15 = "semi-auto rifle"
* M-16 = "assault rifle"
For what it is worth, the rifle used by the so called "DC sniper" was a post-ban clone of the AR-15 and was neither an assault rifle nor an assault weapon.
So there you have it. You wouldn't say someone "pushed in the clutch on their automatic transmission". You wouldn't say they "shifted gears on their single speed bicycle". And you should never use the term "semi-automatic assault rifle".