Definition Series - Part 8: "Semi-automatic"
by Kirk Lawson


One of the most confusing, confused, and misused terms of the firearms lexicon in recent years seems to be the term "semi-automatic" or the short version "semi-auto."

A "semi-automatic" mechanism uses the energy of a firing cartridge to perform the cycle of operation of a gun, once per trigger function. The cycle of operation is 1) extract and empty case after firing 2) load an unfired case into the firing chamber.

In most semi-auto firearms the pressure of the expanding gas is tapped to move a linearly reciprocating part to withdraw the expended and empty case. This is usually a "slide," "breach block," or "bolt" depending on the firearm, often differing between handgun or rifle. The reciprocating part will usually have an "extractor claw" on it which hooks on to the base of the cartridge case in order to pull it out as it reciprocates to the rear while a "ejector" acts as a lever to kick the empty brass out of the firearm's action and away from the machine. As the reciprocating part moves forward again, a new cartridge, under spring pressure, is pushed up in front of a cavity in the reciprocating part and forced forward into the chamber, ready for the sequence to start over again. The function of the trigger is to release the firing pin into the primer, starting the ignition sequence of a cartridge. [See this article for a description of hammer-fired vs. striker fired]


[Kel Tec P3AT firing sequence showing slide reciprocating and loading a new cartridge into the chamber]

Importantly, this sequence only happens once for every single "function" of the trigger. If the firing sequence happens more than once per function of the trigger, according to Federal law 26 U.S.C. § 5845 (b) this would constitute a "Machine gun" or what is sometimes called "fully-automatic" or "full auto." In basic terms, if you pull the trigger once and only one cartridge is fired before you have to pull the trigger again, then it is "semi-automatic." If you pull the trigger once and more than one cartridge fires before you release the trigger, then it is legally a "machine gun."

Another acceptable, though less common, term is "autoloader" or "autoloading pistol." Variations include a space or hyphen separation; i.e.: "auto loader or "auto-loader."

Because of the confusion and often times deliberate misuse or misconstruction of the term to imply "full auto" machine gun into "semi-auto" firearms, please use care with this particular term. There is no such thing as "full semi-auto." Historically, however, some guns, particularly handguns were sometimes referred to as an "auto." This was particularly true of the 1911 pattern semi-auto pistol. In this case "auto" was the shortened slang for "automatic" which was taken from the designation of the cartridge the gun was chambered for: .45 Auto Colt Pistol usually. The term "auto" would be applied, i.e.: "He carried a 45 Automatic" or "The Army issued me an automatic during the Great War" or "His pistol of choice was a big automatic in .45 caliber." The use of the term this way is technically incorrect but common in print literature prior to around the 1990's. If you come across it, please understand that it is being used to refer to "semi-automatic" operation.


[Colt 1911A1 semi-automatic pistol chambered in .45 Auto Colt Pistol (.45ACP)]