AR15: General Specifications and Terminology (Jargon)

Understanding the Functionality of the Ar15 

The AR15 is a gas operated, magazine fed semi automatic firearm. This means that when fired, some of the energy (in the form of expanding gas and pressure) from the fired round is used to cycle the action of the firearm. This is achieved by diverting some of the gas and pressure from a fired cartridge back into the action (bolt key/carrier group)  Through the use of a gas tube. This division of gas and pressure forces the bolt Carrier group rearward into the buffer tube. subsequently the bolt Carrier group is then returned into the upper receiver through the energy of the buffer spring within the buffer tube. As the bolt carrier group returns forward it also strips a cartridge from the magazine (provided one is available) and chambers it for subsequent firing. If no cartridge is available the bolt carrier will lock back if a unloaded magazine is inserted. If no magazine is present the bolt carrier will "return to battery". Being as far forward as possible.
AR15 are capable of Different forms of firing; Semiautomatic, 3 round burst fire and fully automatic fire.  Semi automatic indicates that only one round will be fired from the weapon with each trigger pull. Burst fire is an internal function where 3 rounds are fired with each pull of the trigger then the weapon will cease fire Until the trigger is reset and subsequently pulled again. Fully automatic fire indicates that the weapon will continuously fire when the trigger is pulled to the rear until no ammunition remains or a malfunction occurs.  Further ingenuity has brought a 4th mode of firing known as "binary";  Where not only is a round fired with each "pull" of the trigger, but a subsequent round will be fired with every "release" of the trigger. This Creates a "double tap" effect. 

What is the Ar15 gas system?

The gas operation system of the AR15 comes in 2 major varieties direct impingement and gas piston.  Direct impingement is the aforementioned method of redirecting gas from a fired cartridge back into the bolt carrier group to cycle the action to the rear then back forward. A gas piston system is slightly different it employs the use of a piston system riding above the barrel instead of a gas tube. Gas drives the piston and in turn the piston drives the bolt carrier group.  This method allows no or very little gas to enter the upper assembly or bolt carrier group, keeping the system cooler and cleaner.  though at the cost of additional weight and parts.

The Upper and Lower assembly

The AR15 is divided into 2 major part groups: The upper and The lower. With the upper parts consisting of the upper receiver the barrel and gas tube, the hand guard and bolt carrier group. While the lower portion of the firearm consists of the lower receiver The trigger, the hammer, grip and buffer tube and buffer spring.  The 2 portions of the rifle connect with the use of take down pins.  While the upper portion does contain the barrel which does in fact expel the projectile only the lower portion of the AR15 is considered "the firearm" in legal respects.  Which gives the AR15A unique capability to swap out multiple uppers onto one lower giving users the ability to shoot many different calibers with the need to only purchase 1 "firearm"  Thus needing only to pass one background check.  Every other part of the AR15 besides the lower receiver is an unregulated part and may be purchased without restriction or background check.  For this reason the lower receiver contains the serial number designated to the firearm. 

Ammunition

Though most commonly chambered in .556 x 45mm nato. AR15  Barrels and upper assemblies come in a wide variety of calibers. Some of the more popular chamberings are: 300 Blackout, .223 wyld, 6.5 Creedmore and 300 HAMR.  Normally magazine fed, (however there are belt-fed variations of the AR15) 30 rounds is considered standard capacity.  A loaded magazine is inserted into the magazine well and held in place by the magazine catch.  The charging handle is used to manually cycle the bolt carrier to facilitate the chambering of the 1st round into an unloaded chamber. It is also used in the clearing of malfunctions and to reset the trigger and hammer.