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AR-15 Lower Receiver’s Function and Purpose

Reformed Musings

From the simplest mechanical standpoint, the AR lower’s primary purposes are to position the ammunition in alignment with the bolt and breech of the barrel and position the hammer so that it hits the firing pin. There are numerous other secondary functions of the lower receiver that are critical to the function and usefulness of an AR 15. Let’s go over them!

Fire Control

The AR-15 Lower Receiver houses the fire control functions of the rifle. The safety, trigger, sear and assorted pins and springs work together to provide the movement of the hammer to hit the firing pin when the operator wants it to. The trigger and hammer are positioned in the lower with two pins. There are a number of options for AR triggers. The mil-spec trigger parts are inexpensive, reliable but typically don’t provide the lightest, most crisp trigger pull. There are numerous trigger groups available from inexpensive to very expensive that can provide the type of trigger to meet your specific needs. In a heavy use, like in a lightweight AR shorty, mil-spec trigger groups can work just fine. When trying to compete in NRA HI-Power Match Rifle events, an adjustable, high quality trigger is an absolute necessity. Uses in-between can find many trigger units to fit the specific needs depending on cost and performance requirements. Some trigger groups come in individual parts similar to the mil-spec products and some modern triggers come in a modular unit, which makes fitting very simple.

Buttstock and Grip

The buttstock and grip on an AR-15 lower Receiver are critical to the human element. They are where 2/3 of the contact between the shooter and the weapon occur. The mil-spec stock and grip are inexpensive, very durable and quite functional. Specialized stocks and grips exist in a vast variety, varying tremendously in function, construction, quality and cost. Adjustable length stocks are quite numerous and can be very functional for a number of reasons. The length of pull, (the distance from the butt of the rifle to the trigger), can be adjusted to fit numerous body types. It also allows modern military forces to adjust the length of the rifle to work with body armor and other loads that the modern military is required to carry. Some stocks can also raise and lower the cheek piece to accommodate different sights and optical devices that can be mounted above the existing iron sights, requiring a higher head position to use. The amount that a stock can be shortened is limited by the buffer tube assembly that is housed in most buttstocks. The exception to this is rifles that use a gas piston system to work the action do not require the buffer assembly. Many of the rifles that use the gas piston system have been adapted to side folding stocks, which can reduce the storage size of the weapon considerably. These stocks are commonly made from high grade polymer materials or aluminum. Stocks for “Space Guns” (AR-15’s specifically made for NRA Match Rifle events) have a large range of adjustment to accommodate shooting in prone, sitting and standing positions to provide the optimum fit of the rifle to the shooter in each position.

The Buffer Tube – Energy Conservation

The buffer tube, which attaches to the rear of the AR-15 lower receiver, stores the energy of the bolt flying rearward as the fired case is being ejected from the weapon. A large, specially designed spring and a special “Buffer” are used to absorb this energy to protect the gun from excessive battering from the firing cycle. The stored energy in the spring is then used to strip the next cartridge from the magazine, held in the front of the AR Lower into the chamber, ready to be fired. Different weight and design buffers are used for different applications. Buffers and buffer springs can be changed to match the timing necessary for the rifle to work optimally. The buffer tube assembly absorbs energy, provides the power to reload the rifle at the end of the firing cycle and protects the gun from battering, critical to the performance and life of the AR.

A high quality lower receiver unit is necessary to house all these important functions. Brigade Manufacturing Inc. manufactures a high accuracy US made AR-15 Lower Receiver machined from billet aluminum and Cerakote finished. Photos, specs and ordering information are available at Brigadefirearms

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