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AR Lower Receiver Glossary of Terms

Venturing into the world of buying an AR lower receiver and building your very first AR-15 — can leave you wondering about the terms designated to the AR lower receiver and its components.

In order for you to have a better understanding what is being referred to in AR building verbiage, here is a glossary of terms that relate to the AR-15 lower receiver.

Complete Lower Receiver

– The lower receiver is the AR-15’s viscera. If serialized, it is the only part that is considered a firearm by the BATFE. The term ‘complete lower receiver’ refers to the lower receiver, butt stock, buffer tube, buffer spring, pistol grip, magazine release, and fire control group as an entire assembly. A complete lower receiver is fully functional and will operate as a firearm if attached to an upper receiver.

80% Lower Receiver

– An 80% lower receiver is an unfinished receiver sold by the manufacturer. The percentage of 80% is designated to let the consumer know the milling process has not been completed. On an 80% lower receiver the fire control recess area remains in its original state making the lower inoperable.

An 80% lower receiver can be legally completed by the purchaser and does not require an FFL transfer, serialization or an ATF Form 4473.

95% Lower Receiver

– A 95% lower receiver is much like an 80% lower. Its fire control recess area also remains unmilled and the lower is sold as such. The only difference between an 80% and 95% lower is the upper receiver rear lug pocket has been machined on a 95% lower.

Most manufacturers will designate the term ‘95%’ to let the consumer know the lug pocket has been machined and not as a true percentage. 95% lowers do not require an FFL transfer, serialization or an ATF Form 4473.

Stripped Lower Receiver

– A stripped lower receiver is a lower receiver which has been 100% completed and serialized by the manufacturer. Stripped lower receivers do not include a lower parts kit (LPK) and do require the installation of the LPK, buffer tube and any part required to make the lower operational. Stripped lower receivers also require an FFL transfer and an ATF Form 4473 for purchase.

Lower Parts Kit (LPK)

– The lower parts kit includes the necessary parts to make the lower receiver operate seamlessly with the upper receiver. Most lower parts kits include the hammer spring, selector, bolt catch, magazine release button, buffer retainer and all necessary screws, pins, springs and detents.

Fire Control Group (FCG)

– The lower receiver’s fire control group refers to the trigger, hammer, sear, disconnect, springs and roll pins. There are the parts that allow the AR-15 trigger mechanism to function properly.

Trigger Guard

– The trigger guard rests directly under the fire control group and protects the trigger from accidental discharge.Itis held by roll pins to the ‘ears’ of the lower receiver and is available in a multitude of configurations.

Pistol Grip

– Where legal, the pistol grip serves the purpose of ‘grip manipulation’ for the lower receiver. Modern ergonomic designs also provide better trigger control along with helping the shooter effectively manage recoil.


The buttstock of the AR-15 is available in an array of variations. The most popular being the ‘collapsible’ buttstock. The buttstock is attached at the lower receiver’s buffer tower or buffer tube and provides the shooter with a stable shooting platform when the rifle is shouldered.

For more tips on building your own AR-15 or how to choose a lower receiver, be sure and check out our resource center where you can learn more about building your own AR-15.  

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