The ultimate AR 9mm build guide

This article is the result of me wanting to build a PCC AR pattern rifle pattern rifle and the resulting outcome of my experience. I want to point out that there are some sections of this article that apply only to specific gun owners in the United Socialist States of America (i.e. CA, NY, etc.). That being said I do cover the same questions and build requirements that every AR9 builder will need to know so if you live in freedom loving America, just skip the sections marked in RED

(I have to put a legal disclaimer here that I am not a lawyer and nothing in this article should be considered legal advice.  You are responsible for knowing all laws relevant to your situation and if you have any questions please reach out to a lawyer and/or law enforcement)


So obviously in order to build a 9mm AR you will need a barrel chambered for 9mm.  The barrels are different than normal AR barrels in that there is no gas port which means no gas block is required.  This is because PCC ARs are direct recoil operated, not gas operated.  As such there are a few things different about building an AR9 clone.  The good news is that there are some similarities as well.  The upper receiver, LPK (in some cases the FCG may need to be changed out), furniture, and buffer tube are the same between a standard AR15 and the AR9.  This article was written to examine the differences so I will not be going into more detail about the similar products.


Lets start off with the “Why would I want to shoot pistol ammo out of a rifle?” question.

With the cost of rifle ammo, and especially the new laws restricting how Californians can purchase it, shooting pistol caliber cartridges out of a carbine is pretty appealing. With lower cost, more readily available, and easier to reload ammo it makes a lot of economical sense to want to shoot 9mm out of an AR platform. True you will not be able to make 600 yard shots with it, but then again there are so few places to actually shoot that far anyways that most of us are limited to 100 yards or less which is absolutely in the sweet spot for PCC ARs.


Before you start buying parts you need to decide what lower you are going to use.

There are 2 questions you need to ask yourself before you embark on your AR9 build.  What style mags, and do you want, and do you want to use a dedicated lower or mag adapter?  The 1st question is fairly simple.  You can choose Glock or Colt pattern mags (you could also use UZI mags if you modify

This is a Glock pattern AR9 lower from TN Arms Co. Notice the mag release. A dedicated Glock lower will typically not require you to modify the magazine but a mag adapter will

them with Colt style lowers).  If you own a Glock 17, then its a no-brainer.  If you are like me and don’t own anything Glock then its more a matter of cost which will probably end up being Colt.  What is not always so easy to decide is if you want to use a dedicated lower or a magazine well adapter.

Deciding to use a dedicated lower for your AR9 build or a mag-well adapter is dependent on a couple of things.  If you have a spare lower laying around and don’t have any plans for it, then the mag-well route will be the lower cost option.  It will also be a good choice if you only have 1 lower and  want to be able to swap uppers depending on the need.  Just keep in mind that not all mag-well adapters are the same.  The Glock adapters tend to be more money than the Colt pattern ones, and they require you to modify your Glock mags by cutting a notch in them.  Since the original 9mm AR used used Colt SMG magazines, going the Colt pattern route will have mags that require no modification.

Restricted State Builders: 

  • Be aware that you must choose at this point if you want to build a “featureless” or fixed mag rifle.  If you want to go fixed mag and build a dedicated Glock mag lower you will need to buy a special mag lock for the Glock lower mag release.  If you use a mag-well adapter (either style) you can use a normal AR15 mag lock device and it will work just fine.  This will add to the cost of the build so make sure you account for it.  I choose to go the featureless route because it was cheaper and there is no Mean Arms loader for 9mm to make reloading fast.
  • It may be best for those living in states that restrict magazine capacity to less than 10 rounds to go the Glock route.  The smallest Colt SMG mag I have found is 10 rounds which means it will not be allowed for some states.  Since Glocks are one of the most common handguns in the world, you will have an easier time finding smaller capacity mags.


Now that we have our lower picked out, what else is required?

The next thing we need to decide on is what buffer to use.  I know this sounds like a silly question, but it definitely will make a difference.  The thing to remember is the longer the barrel, the more weight you will need.  The original AR9 buffers are the same length as normal AR15 carbine buffers, but weigh 5.5 oz.  This is because a PCC AR uses direct blowback to cycle the bolt as opposed to a gas system.  That means its force of direct recoil that is sending the bolt back, and the buffer weight/spring sending it forward.  If you don’t have enough weight/spring tension to slow it down you will have problems.

This is the only manufacturer data aligning AR9 barrel length with buffer weight. You may be able to use a lighter buffer in your build but if you follow this table you will most likel be goo to go.

The weight of the buffer you need to use depends on your setup.  My build is a 16″ barrel so the recommendation is I use a 8oz buffer at a minimum.  The same recommendation goes if you are running suppressed.  For shorter barrels, the amount of recoil force is not as great so you can use a lighter buffer weight.  Regardless the barrel length to buffer weight config, you need to make sure you shorten the travel of the buffer.  If you do not you will break your bolt catch (if you have one) and open yourself up to cycle issues.  Your options are to either buy an extended length buffer or You can do this by stacking 7 quarters together and dropping them in the buffer tube before putting the spring in, or you can buy a buffer spacer like this one.


Mag well adapters

OK we are back on the Glock v Colt style mag conversation again.  Like I stated earlier, the Colt styles have been around longer, so they have the kinks worked out of them, and offer LRBHO function.  The Glock style adapters are appealing because Glock mags are more plentiful, and arguably more popular.  Most people have a Glock something in their collection and as such have a collection of mags already.  The downside to the Glock style adapters are:

  1. No LRBHO
  2. Require you to modify the mags to work with the AR lower in most cases
  3. Not as reliable as the Colt style due to newness to market
  4. Usually more expensive than the Colt style adapters
This is the Pro Mag colt style adapter for AR15 lowers

If I owned a 9mm Glock handgun I would have gone the Glock route most likely, but since I do not it made more sense to me to go the Colt style route for the reasons listed above.  I ended up buying Pro-Mag’s Gen 2 adapter (part number pm237b) and it works like a champ with both Pro-Mag and ASC branded 10 round steel magazines.


What about the Bolt Carrier Group?

The BCG is another component to pay attention to.  Besides the obvious difference in looks, you need to make sure your BCG will work with your lower.  There is a BCG that is specific for Colt mags, as well as one specific for Glock

This is the BCG that came with my upper. It is compatible with both mag styles and standard AR15 FCGs.

mags.  Beyond the magazine type compatibility, you also want to make sure that the BCG is ramped to be used with a standard 5.56 AR 15 trigger.  If it’s not, then you will need a special FCG which as we all know means extra cost for your build.  The good news here is that most 9mm BCGs are whats called a hybrid BCG meaning it will work with both style mags as well standard AR 15 trigger groups, just be sure to check before spending your hard earned cash.


Know your gun, details matter!  Not all 9mm muzzle devices are the same.

Another small detail to pay attention to when building a PCC AR is the muzzle device threading.  For 9mm builds the barrel will typically be threaded either 1/2×36 or 1/2×28.  1/2×36 is a 9mm specific threading, while the 1/2×28 is the standard threading for 22 cal muzzle devices.  BE CAREFUL if you have a 9mm barrel with a 1/2×28 muzzle as you will be able to attach a 22 cal break/hider/comp/etc. but it will not go well when you try to shove a 9mm bullet through a 5.56mm hole.  Likewise a 1/2×36 device will not thread to a 1/2×28 barrel (and vice versa).

Restricted State Builders:

For those of us in restricted states looking to build a “featureless” AR rifle, you will most likely need to replace the muzzle device that comes on your upper if it is pre-assembled.  I have seen a couple of uppers with breaks but most come with the A2 flash hider installed so be prepared to swap them out.  If you are unsure of the barrel threading call the manufacturer and ask before spending your money and risking buying the wrong thing.


Last but not least it’s a good idea to upgrade the trigger and hammer pins.

You can find a set of anti-walk pins on eBay for cheap. These are the ones I used on my build

For about $10 you can pick up a set of anti-walk trigger pins.  I have mentioned it before but the fact that an AR9 is direct recoil driven and not gas operated means the internal components will take more of a beating.  While the standard trigger and hammer pins you get in your average AR15 LPK will work, it is strongly recommended that you use some stainless steel anti-walk pins.  You probably won’t need them to be honest, but for the price think of it as extremely cheap insurance.


I hope this helps clear up some of the confusion/ambiguity around building a PCC AR.  I love shooting my 9mm AR.  It is low recoil, cheap to feed, and a lot of FUN!

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