The AR9 is a wonderful thing. Since it is a part of the AR platform, it allows users to take advantage of the wide variety of parts that any AR-15 or AR-10 could also use. Given its extreme modularity, the AR-9 can also fulfill several different roles. For example, as a pistol caliber carbine (PCC) it can excel as a personal defense weapon when built out to be a small and compact package. But it could also be fitted with a full length, 16” barrel for shooting competitions as well.

While not as immediately popular as the AR15, AR9 build kits have become increasingly popular and more widely available than before because whether there is an ammo shortage or not, it just makes sense. If you’re thinking, “why build an AR-9” go check out our previous article on that. This blog, in particular, is your guide to AR9 parts selection when building an AR9 kit from scratch.

What is an AR9 build kit?

The term ‘build kit’ or ‘conversion kit’ gets hung up a lot and can be used interchangeably to mean a couple different things. With regard to AR9 setups, we’d like to clear that up for you right now. One one hand, an ‘AR9 build kit’ might mean that a single purchase will get you everything you need from the AR9 lower receiver, AR9 upper receiver and all the AR9 parts you may need to fully assemble your pistol caliber carbine. But the term ‘build kit’ is used quite loosely so it might only be referring to all the parts for either the upper or lower receiver and not necessarily both. However, if the word ‘complete’ is used in the product description that can be pretty telling for what parts you can expect it to include.

Sometimes, ‘conversion kit’ simply refers to a complete upper receiver for a specific caliber which allows upper receiver swaps while using a complete lower receiver you already owned. Other times it might mean you’re sort of frankenstein-ing one rifle build to shoot a different caliber not originally intended.

AR 9 Lower Receiver

An AR9 lower is most easily identified by its noticeably smaller size or the magwell converter that may stick out if you’re using an AR15 lower as the base. However, there are a few different types of lower receivers you could get for your AR 9 build including, but not limited to, 80% lower receivers, conversion kits or 100% stripped AR9 lower receivers.

AR 9 80 Lower

Currently, Modulus Arms does not offer an 80% AR9 lower receiver but you can expect to see some in the near future as we’re currently working on a new design with our signature curved lowers that have increased, flared mag wells.

For the same reasons of building any 80 lower, making your own AR9 build using an AR9 80 lower is a good opportunity to make your AR9 build more personal than buying a stripped lower via an FFL. Having a blank AR9 lower receiver to start means having a perfect space for any personal engravings where a manufacturer would have put their own logo and serialization.

Your dreams of making the ugliest or coolest AR lower with the serial number of 420XBLZNG can be realized with an 80 lower kit. One of the primary benefits of an AR9 80 lower is that the lower comes without having to involve any government agencies for those who prefer that kind of discretion. Despite the additional cost of time and money for an AR9 80 lower, this kind of project is right up the alley for those with a knack for putting things together.

AR 9 Conversion Kit for AR 15

The answer to most AR-15 owners who want to try a new caliber, such as .22 LR or 300 BLK, tends to involve buying just a new upper or bolt carrier group in a different caliber while using the same lower receiver. You can do the same for a 9mm complete upper receiver. This is a cost-effective measure and swapping AR upper receivers or upper receiver parts takes less than ten seconds to do.

When doing upper swaps to get an AR9 with an AR15 lower, you’ll have to buy a conversion If you don’t want to buy a dedicated lower for the AR9 and there are plenty of AR-15 conversion kits to choose from! We would choose a conversion kits made by reputable brands such as Hahn Precision’s 9mm Drop-In Block or Stern Defense’s AR-15 9mm Conversion Adapter wich can easily outfit your AR-15 lower to be able to receive 9mm magazines. For your convenience, these are made and offered for both Colt and Glock magazines.

AR9 Magazine Conversion

If you’ve been planning on picking up some Glock magazines regardless of whether or not you are getting a Glock magazine fed AR9 (or you don’t mind altering your Glock magazines), there is the TorkMag Magdapt™ 17 conversion kit. The conversion kit itself starts at $59.95 as opposed to Stern Defense’s $179.99 - $186.99 and Hahn Precision’s $200.00. The Glock magazines you use with the TorkMag MagAdapt must have an AR magazine catch recess which can be cut into the magazine, or you can buy the TorkMags that work just like Glock magazines in a Glock. A five pack of 20 round TorkMags can be had starting at $94.75. While effective, the TorkMag MagAdapt conversion does not feature LRHBO. If proprietary magazines aren’t an issue, the TorkMag is an excellent option for the cost.

MEAN Endomag 9mm

Another magazine conversion kit that features the LRBHO function is the MEAN Endomag 9mm. This kit will convert Gen 3 and Gen 2 Magpul PMAGs to hold 9×19mm Parabellum, allowing you to feed the converted magazines into a standard AR-15 magazine. Starting at $29.99, the kit will run you up to around $40 after shipping and taxes. Shooters can use a converted magazine in combination with a normal AR-15 lower receiver and an AR9 upper receiver to have a functioning AR9.

Although this option is the least expensive alternative so far, it does come with one safety drawback that must be taken into account especially if you intend to share this firearm with someone else. As with most lower receivers, the ejector of an AR9 is typically pre-installed on the AR9 lower. For this magazine conversion kit, the ejector will sit on the magazine of the feeding lip. When you remove the magazine, you also remove the ejector. The round will stay on the bolt carrier group regardless of how hard you charge the handle so long as the magazine is removed.

In the event that your buddies (or even you) aren’t used to the round not ejecting after the magazine is removed, the user thinks that they’ve cleared the firearm when they’ve unwittingly just sent the round back into battery after chamber checking. If this risk is minimal to you, then by all means continue. However, it is a safety risk to always consider and prevent from happening with stringent communications before anyone else touches your AR9 with an Endomag.

100% Stripped AR9 Lower Receiver

Although it might not be our jam, it very well may suit you just fine to buy a 100% stripped AR9 lower receiver. Buying this would likely mean subjecting yourself to a 4473 form, meaning that you’d need a background check to buy this lower. If you don’t mind that, Aero Precision’s EPC-9 lineup is pretty great. We’re big fans of Aero Precision for their wide variety of AR parts that offer great finishes and tolerances.

LRBHO Function

An AR9 lower is most easily identified by the smaller, curved magwell compared to an AR15 lower. This magwell design uses less material but isn’t significant enough to provide any noticeable weight saving. However, this magwell design facilitates faster reloads than that of a converted AR-15 lower with a flared lip. Your AR9 80 lower will typically come with a fin-shaped ejector pre-installed along with a magazine release. The reason for this is that the fire control group for a 9mm is much smaller compared to that of 5.56 NATO.

LRBHO (Last round bolt holds open) describes the bolt carrier group staying in the rear locked position after the last round has been shot. This is a common feature that shooters use to quickly check if their firearm is out of ammo without having to cycle the gun. While those who come from a background of shooting AKs, the LRBHO allow shooters the convenience of not ejecting a perfectly good round every time in case they aren’t really empty when chamber checking.

When milling out an AR9 80 lower, you’ll notice that the result of the direct blowback design is the bolt carrier group positioned in the front further from where the bolt catch is normally on an AR15. To compensate for this, the bolt catch is typically integrated into the AR9 upper receiver rather than the AR9 lower receiver. What this means is that you won’t have to worry about the bolt catch until you choose an upper for your AR9. The early renditions of the AR9 did not feature LRBHO, but it is common now on today’s market of AR9 upper receivers.

Are AR15 lower parts kits compatible with AR9 lowers?

Yes! The trigger space for the AR9 and the trigger space for the AR15 are the same. You can use the same triggers for an AR15 with an AR9 lower along with the same safety selectors, pistol grip, buffers, and trigger guard. Some pistol grips depending on brand may require the use of spacers during installation.

For buffers in combination with a 4” to 8” upper (as typical of an AR9 pistol carbine), a buffer of around 3.8 oz will help your AR9 run smoothly with a sturdy buffer spring such as the shot-peened, heat treated chrome silicon wire Sprinco Blue. With a weak buffer spring and a buffer of less than 5 oz, you could find your spring snapping into pieces during operation. Although heavier buffers can be found with some going as heavy as 10 oz, users might see a failure of the bolt carrier group to return to battery with buffers heavier than 5.5 oz so expect some trial and error.

AR 9 Glock Lower vs AR 9 Colt Lower

When choosing an AR9 lower whether it be a complete AR9 lower or an AR9 80 lower, the magwell fitting are designed for either exclusively to colt magazines or exclusively to glock magazines. Glocks having one of the largest cult gathering and extensive aftermarket accessories mean you’ll find no shortage of Glock mags so long as you are on the US continent. For this reason, most people go for the Glock fitted AR9 lower as being able to share magazines between their Glock handgun with their carbine is a very convenient feature. But if you enjoy the old school feel of metal Colt magazines, you’ll be happier with a Colt AR9 lower.

Benefits of Building an 80% AR9

Although both an AR9 lower and an AR15 lower can be used to complete an AR9 carbine or pistol, there are certain advantages a dedicated AR9 lower holds over using a converted AR15 lower. Shooters who want an AR9 typically want an AR9 in a small package, not a full sized rifle. This is where using a conversion kit can run into problems.

By federal law, someone who bought or transferred their lower as a rifle cannot legally fashion their lower into a pistol configuration. Keeping an AR9 as a full sized rifle is about as practical as it sounds. If you got your AR-15 as a rifle like most shooters, this is where having an AR9 lower registered as a handgun starts to get practical. Which is another reason why it’s just easier to get a 80% AR9 lower to skip that headache.

Regulations aside, the practicality of having an AR9 lower as opposed to a converted AR15 lower can be pointed out in the fact that the AR9 shares few parts with the AR15. With the exception of trigger assembly and some lower parts, the most expensive parts are not interchangeable. You still need a BCG, barrel chambered in 9mm and caliber specific muzzle device just to name a few.