Photo above:  AR15.com

America has plenty of two things: 9mm and ARs. It doesn’t take much to consider if we could combine the two staple commodities together. The pistol brace already exists as a major point of contention between the gun community and the ATF so why wouldn't you make the AR pistol into an actual pistol caliber carbine?

What gas system does the AR9 use?

Straight blowback action is the most common action for non-large pistol calibers. It’s an action that relies entirely on the expanding gas in front of the bolt carrier group to operate the action. While a set of locking lugs on the AR15’s bolt carrier group dampens the initial pressure build up, the AR9 bolt carrier group has none. The straight blowback design makes the AR9 bcg take on the entirety of the gas pressure all at once with no dwell time.

To prevent the bolt carrier group from opening prematurely, before the bullet has left the barrel and vented out the pressure behind the projectile, the AR9 bolt carrier group must have more mass than the standard AR15 bolt carrier group to work against the force of pressure. Otherwise, the brass casing will rupture violently and give you an out of battery discharge. The bigger the caliber, the heavier the bolt carrier group.

For anyone who has held a Thompson, you’ll quickly understand why it is not practical to develop a straight blowback firearm in anything more than .45 ACP. An extremely heavy reciprocating mass during operation does your aim no favors. While a heavy bolt carrier group mitigates velocity, there is no practicality in lugging around a brick with more cumbersome weight than firepower.

Can I make my own lower for an AR9?

We have a dedicated AR9 80 percent lower available in hard coat anodized black, FDE, and raw aluminum. These milspec lowers accept glock magazines and can be enjoyed with a LRBHO appropriate upper receiver.

Could I use an AR15 lower for an AR9?

If you feel inclined enough to use that AR15 lower that has been hibernating in your closet since the fall of the Ottoman Empire, you can use an adapter to equip an AR15 lower to feed 9mm magazines.

Can I use an AR-15 buffer for an AR-9?

Having established the AR9 bolt carrier group is heavier than that of the AR15, it’s reasonable to question how this could impact the buffer system. An AR9 uses a buffer that is noticeably longer than the AR-15 buffer. The reason for this is because the AR9 bolt is ¾ of an inch shorter in the front than that of a standard AR15 bolt. An AR9 bolt running on a regular, shorter AR15 buffer will recede too far back into the buffer tube and leave enough room for a 9mm casing to fall into the trigger action of the AR9, rendering the trigger inoperative.

Another reason for not allowing the AR9 bolt to recede too far back is to prevent the bolt carrier group from striking the bolt catch and smashing it open, a common result of using a standard AR15 buffer in an AR9 system.

For a direct blowback AR9 in selective fire, a bolt with sliding weights is important to prevent the bolt bounce that leads to out of battery detonation. Simply shake your bolt and you’ll know the sliding weights are there when you feel them rattle.

Make sure to use a proper 9mm buffer to avoid these malfunctions. Altogether your bolt carrier group and buffer should weigh within 20 oz to 22 oz as much as possible when shooting factory loads.

Stern Defense SD BU9

Stern Defense’s 9mm bolt has earned the title for the most budget friendly AR9 bcg, sitting at $129.99. The SD BU9 comes with a standard mil spec magnesium phosphate finish and accepts both Glock and Colt magazines. This is a solid choice for the feeling of a crisp single stage trigger reset.

• 15.5 oz with weight
• 13.3 oz without weight

Faxon Gen2 9mm Bolt Carrier Group

For those who prefer a cleaner heat treated nitride finish and want to retain the last round bolt hold open feature, Faxon’s Gen2 9mm bcg can be had for $156.75. It plays well with a standard AR-15 upper receiver and dust cover.

• 14.9 oz

Angstadt Arms 9mm Buffer Kit

Anstadt Arms offers a buffer system in the proper AR9 length and weight of 5.4 oz with their 9mm Buffer Kit. This is a solid buffer available for $54.00.

JP Enterprises Silent Captured Spring

The full stroke Silent Capture Spring is going to be one of the most effective springs from JP Enterprises when pairing with non-JP bolts. They will help manage the felt recoil that direct blowback operation is known for, but keep in mind that you must remove the added internal weight of your AR9 bolt for the pair to function. The guide rod of the Silent Capture Spring runs through the hollow channel of the bolt. For this reason, the Silent Capture Spring tends to be on the heavier side, 9.2 oz, to compensate and meet that 20 oz to 22 oz target weight. This buffer runs for $187.10 but can be worth checking out if you’re going for a smooth, suppressed system.

Odin Works Adjustable 9mm Buffer

If you found a 9mm bolt that you’d like to use but can’t find the correct weight buffer, try using one of Odin Work’s adjustable buffer for $60.80.

What is the best length for an AR9?

9mm does not gain relatively much from a longer barrel and barrel lengths longer than 9” will only add minute velocity gain at the expense of less maneuverability and added weight. Staying with at least a 7” AR9 barrel makes for good cycling and improved grouping for the typical 115 gn and the subsonic 124 gn.

Faxon 8.5” Light Tapered 9mm Barrel

Faxon’s 8.5” nitrided 9mm barrel is a good example to use for a AR9 carbine build and comes at $119.99. The 1:10 twist is fantastic for stabilizing heavier grains of 9mm should you want to follow a suppressed subsonic AR9.

Hahn Precision 9mm Adapter

Hahn Precision’s AR-15 9mm Conversion Block features a two piece steel and aluminum construction adapter to feed Colt magazines and modified Uzi magazines. The bolt catch comes pre-installed in the dedicated version of their conversion block. It is currently available for $200.00.

• Colt Magazine Fed
• Last Round Bolt Hold Open

Stern Defense 9mm Adapter

If you don’t feel like paying the premium for Colt magazines to achieve the authentic aesthetics and you also happen to have plenty of Glock magazines for America’s staple polymer handgun, try Stern Defense’s 9mm Conversion Block for $179.99.

• Glock Magazine Fed
• Last Round Bolt Hold Open
• Accepts All Generations of Glock Magazines

Stripped AR9 Lowers

Need a dedicated AR9 lower but don’t mind going through an FFL? Getting a stripped lower directly may be a better option for you.

AR9: Colt Magazine or Glock Magazine?

Colt’s SMG magazines enjoy a battle-proven history in PDWs as double fed, straight facing magazines that can handle Marine-tier abuse. For those who want the nostalgia and feel of the original AR9, running with Colt magazines can hit that spot.

However, a 32-round 9mm Colt magazine runs twice the cost of a 33-round Gen 5 Glock magazine. It’s not a bad idea to be like a Kriss Vector owner and share magazines with a Glock if you already have one or would like to limit expenditure.

Build Your Next AR with Modulus Arms!

Let us help you build an AR lower in any of the staple flavors of caliber you could want. Feel free to check out our wide variety of 80 lowers for  AR15, AR10, and AR9!