Serial Numbers – The Difference Between AR 15 Uppers and Lowers

For those of you who are new to the world of AR 15s, let’s go back to basics. To start with, let’s clarify something so you don’t accidentally put your foot in your mouth when it comes to this rather notorious firearm: the “a” and “r” of AR 15 does not stand for assault rifle. It is actually shorthand for “Armalite rifle” after the small arms engineering company who initially developed this firearm style; the 15, as you may have guessed, designates the AR 15 as the 15th firearm design Armalite created. Now, since we’re talking about the very basics of the AR 15, we ought to clarify something else as well. Let’s talk serialization and legally selling AR 15 parts and accessories.


The Basic Components

Lower Receiver

The AR 15 is divided into two parts, the upper receiver and the lower receiver. The AR is divided this way because only one portion is the actual firearm component. The lower receiver is the part of the AR 15 that contains the fire control group (the trigger, disconnector, hammer, and fire selector), so this is the only portion of the AR that must be marked with a serial number, manufacturer’s name, and so on. Because of this, the lower receiver is the only portion of the AR 15 that must be sold through a licensed firearm dealer.


Upper Receiver

The remaining portion of the AR 15, the upper receiver or upper assembly, is what most AR enthusiasts are referring to when they talk about the versatility and customizability of the AR 15. The upper assembly is modular in design so it is easier to swap out everything from the charging handle to the barrel and rail system. The modular design means any AR 15 owner can, after purchasing their lower receiver through a licensed seller, choose between purchasing a full upper receiver parts kit or shopping around for the best AR accessories and parts to create their own upper assembly.


The Benefits of the Modular Design

Because the AR 15 was created with two separate components, that opened the door for a great deal of freedom when it comes to choosing components and customizing the firearm in a myriad of ways – but it also allows for a lot more freedom of manufacturing. Since the lower receiver is the only component that needs to be serialized and purchased through a licensed dealer, any of the the upper assembly components can be designed, modified, and manufactured by anyone with the engineering skills and fabrication sources necessary.

Essentially, the AR 15 as a firearm design supports small businesses rather than requiring every piece to go through one of the big name firearms producers. A small business can focus on creating one (or a few) AR 15 components rather than everything, which brings about the rise of specialty shops. On top of that, upper receiver components can legally be sold online as well as in stores, so those small specialty shops don’t have to face all the start-up costs associated with opening a physical location – and that savings gets passed along to our customers!


If you’re looking for upper assembly components, from a complete upper receiver to an adjustable gas block, shop Bootleg, Inc. today!


3 thoughts on “Serial Numbers – The Difference Between AR 15 Uppers and Lowers”

  1. I was looking at an upper that I’m currently doing a build on and I noticed that it has a number on it. I turned the upper over and looked inside along the underside of the picatinny rail where the charging handle slides and there is a clearly identifiable number etched into the receiver. Is this unusual?

  2. I have been trying to figure out what makes the AR lower the serialized part. The comment ” The lower receiver is the part of the AR 15 that contains the fire control group (the trigger, disconnector, hammer, and fire selector), SO this is the only portion of the AR that must be marked with a serial number, manufacturer’s name, and so on.” Doesn’t really explain it. I have been putting together rifles for a long time. Everyone knows that the AR lower is the serialized part and I never really questioned it. Most also know that the lower doesn’t really legally qualify as a firearm because it does not interface the barrel or hold the bolt carrier group, all of which fall under the legal definition of a firearm. It wasn’t until I built up a few other types of guns that the questions arose. For instance the HK UMP 45 has a similar architecture to the AR-15 with the lower receiver holding the same above mentioned parts and the upper receiver interfacing the barrel and holding the bolt carrier group. Yet for the UMP the upper receiver is the serialized part and therefore the part that must be go through an FFL. So is it really just arbitrary? Does the original manufacturer or patent determine which half is serialized? If I am manufacturing AR from scratch is it technically to my discretion as to whether it is the upper or lower that gets a serial number? As which most gun restrictions I understand they is a lot that is unclear but was wondering if ya’ll can provide and clarification.

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