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SDI School of Firearms and Technology AR-15 course [Shared]
Transcript of SDI School of Firearms and Technology AR-15 course [Shared]
School of Firearms and Technology
Parts and Functi n
Exploded view of
lower receiver on AR-15
The lower receiver houses most of the fire control group and part of the action. It is:
The centralized component of the AR platform rifle.
The only serialized part of the AR platform rifle.
Technically considered to be the firearm itself.
One of the only standardized components of any variant or configuration of the AR platform firearm.
This course is designed to teach you how to assemble, disassemble, clean, diagnose and repair the standard AR-15 carbine chambered in 5.56x45 (.223 remington).
These techniques and procedures can be applied to the majority of the AR platform variants and configurations.
When finished with this course, you will be able to maintain AR platform rifles and keep them in service longer.
Components of the fire control group
The fire control group is the collection of parts, that when actuated create ignition in the cartridge.
Direct connection between the operator and rifle.
Holds the hammer in the cocked position.
Releases the hammer to begin ignition.
Strikes the firing pin with enough force to crater the primer.
Relies on mass and velocity to create enough energy for reliable ignition.
Regulates the semi-automatic function.
When the trigger releases the hammer and the rifle fires, the bolt carrier will travel rearward and cock the hammer. If the trigger is still depressed, the disconnector will catch the hammer and prevent it from traveling forward and striking the firing pin. When the trigger is released, the disconnector releases the hammer and the fire control group will reset. Once reset, the rifle is ready to fire again.
Safety Selector Assembly and Pistol Grip
Ambidextrous selector switch
The safety selector is used to select the mode of operation for the rifle. Here are the different modes and their functions:
SAFE: The trigger is physically blocked from moving and releasing the hammer.
FIRE/ SEMI, the trigger is no longer blocked and allows the rifle to fire in semi-automatic mode.
The pistol grip is an integral part of the selector assembly because it retains the selector detent and spring.
Magazine Catch Assembly
The magazine catch assembly serves to lock the magazine in position in the lower receiver.
The magazine catch button is depressed to unlock the catch and release the magazine.
Magazine catch assembly
The bolt catch assembly:
• Serves to lock the bolt back in the receiver, opening the breech.
• Can be operated manually or automatically.
• May also be initiated to clear malfunctions.
Bolt catch assembly
How a bolt catch assembly functions:
A shelf on the rear of the magazine’s follower pushes against a tab on the bolt catch when the magazine is empty.
When a full magazine is inserted, the bolt catch is depressed, releasing the bolt and loading the rifle.
Takedown pin assembly. The pivot pin assembly is almost identical except the pin is longer.
The pivot and takedown pin assemblies secure the lower and upper receivers to each other.
When the takedown pin is pulled out, the upper receiver will pivot on the pivot pin and allow access to the bolt carrier assembly and charging handle. When the pivot and takedown pins are both pulled out, the lower and upper receivers can be separated for service and storage.
Pivot and Take Down Pin Assemblies
The stock assembly:
Provides a support to stabilize the weapon against your shoulder.
Acts as a rest for the cheek.
Contains the buffer tube, which in turn contains:
Buffer spring: the recoil spring that closes the action and assists in locking the breech.
The buffer: helps the buffer spring with the absorption of energy from the momentum of the bolt carrier group cycling to the rear.
How they work:
The bolt carrier group cycles back into the buffer tube inside the stock, compressing the buffer spring.
The energy of that motion is absorbed by the compression of the spring and the weight of the buffer.
The stored energy is released to push the bolt carrier group forward.
Adjustable stock assembly
Length of pull can be set at different lengths with the adjustable stock assembly. Parts differ between the rifle and carbine stock assemblies.
The trigger guard assembly serves to protect the trigger from unintentional contact.
Trigger guard assembly
Upper Receiver Assembly
The upper receiver:
Serves as the housing for many components.
Is the mounting point for the lower receiver, front, and rear iron sights.
Can be configured with different length and caliber barrels and handguards.
Other optics can also be mounted to the a4/M4 flat top upper receiver.
Stripped upper receiver
Barrel, Handguard, and
Gas System Assemblies
The barrel assembly serves:
To contain the pressure of the cartridge during discharge.
As an attachment point for the gas block, handguard, and muzzle device.
Attaches to the barrel nut and with some models, to the barrel.
Provides a support point for the non-firing hand.
Provides an attachment point for sights and accessories.
The gas system directs high pressure gas through:
A hole in the barrel (gas port) into the gas block.
The gas tube and into the gas key (on the bolt carrier).
This gas is used to cycle the action to the rear. On some variants the front sight base is integral with the gas block.
There are four lengths of gas systems:
The bolt serves multiple purposes in the rifle:
It feeds and locks the cartridge in the chamber.
The bottom two bolt lugs strip rounds from the top of the magazine.
It features a built-in extractor and ejector.
After discharge, the bolt will extract and eject the spent case from the ejection port.
The bolt carrier assembly performs several tasks in the cycling process. It:
Transfers energy from both gas pressure and the recoil spring into the action cycling.
Locks and unlocks the bolt in conjunction with the cam pin.
Cocks the hammer.
Can be manipulated manually by the charging handle.
Bolt carrier assembly
The forward assist assembly serves to push the bolt carrier forward, locking the bolt. This is not always necessary, but is useful when a round does not feed completely into the chamber. Not all variants feature a forward assist.
Forward assist assembly
The ejection port cover assembly:
Covers the ejection port to keep dirt and other unwanted debris from entering the firearm.
Opens automatically when the bolt travels past it, allowing spent casings to be ejected.
Ejection port assembly
The charging handle assembly is:
Used to manually open the breech or cycle the action.
When manipulated, the charging handle pulls against the bolt carrier, unlocking the bolt and opening the breech.
When released, the bolt carrier and charging handle travel forward under recoil spring pressure, closing the action and locking the breech.
Useful for loading, unloading, and clearing malfunctions.
Non-reciprocating (does not move with the bolt carrier under normal cycling).
Charging handle assembly
The magazine assembly serves as the feed source for the rifle. The AR-15 (stanaG 4179) box magazine arranges cartridges in a double stack/double feed configuration:
Cartridges are secured and aligned by the feed lips.
The follower supports the rounds and guides their arrangement.
The magazine spring pushes the follower, forcing the cartridges upward in preparation of feeding.
There is also a shelf on the follower that engages a tab on the bolt catch, which locks the bolt back when the magazine is empty.
Front Sight Base
The front sight base:
Houses the front sight.
Is adjustable for elevation.
Front sight base
Rear Sight (A1/A2)
Rear sight variants:
Some variants may be equipped with a fixed carry handle and a rear sight assembly.
The a1 variant is adjustable for windage.
The a2 variant is adjustable for both windage and elevation.
The a4/M4 variant comes equipped with a detachable Picatinny carry handle with an a2-style rear sight assembly.
A1 sight assembly.
A2 sight assembly.
Cycle of perations
There are eight cycles of fire:
AR 15 animation
Stripped lower receiver
This collection of parts includes the:
Bolt Catch Assembly