Backpack Guns – Rock River Arms RUK-9BT and RUK-15

First Look: Rock River Arms' RUK-9BT and RUK-15.

Backpack Guns – Rock River Arms RUK-9BT and RUK-15
In terms of size and ergonomics, Rock River's new RUK-9BT is similar to the RUK-15. Functionally, however, there are several differences. (Photo by Mark Sidelinger)

If you have to ask “Why?” you’re probably not in the market for Rock River’s new RUK-9BT and RUK-15. These are specialized, semiautomatic pistols for personal defense — and I’m being serious. From the standpoint of a former Marine who has been caught up in a gunfight, a pistol is good to have, but a firearm that you can shoulder and manage with three points of contact is more effective.

The problem with many rifles having 16-inch or longer barrels is that they can present maneuverability challenges inside tight spaces, and they are difficult to store or carry if you need to be discreet. Short-barreled rifles (SBR) are a solution, but for law-abiding citizens there is delayed gratification caused by the process of submitting an ATF Form 1 for the manufacture of a firearm or Form 4 for the transfer from a dealer to an enduser. According to www.nfatracker.com, the average wait time for approval for an individual is about 8 months.

Rock River Arms
The RUK-15 measures 23.5 inches with the SBA3 arm brace extended, which means that it could be discreetly transported in a backpack or similarly sized enclosure. Here it is shown in the fully collapsed position with the provided 30-round RRA magazine and back-up sights flipped up.

There are also jurisdictions where state law restricts or prohibits SBR ownership, leaving its citizens with the option of acquiring a pistol variant equipped with an arm brace. The ATF has previously ruled that contact between an arm brace and the shoulder does not reclassify such a pistol as an SBR so long as the arm brace isn’t altered from its original design.

And this brings us to the practicality of an AR-pattern pistol set up with an arm brace for use as a backpack gun for self-defense. Often, the law requires us to transport our firearms in an enclosed case anyways, so a backpack makes sense as well as quick-access solution to ready a more effective firearm than a sidearm would be if you were ever to find yourself in a critical situation and had time to retrieve it.


Don’t think you’d benefit from having quick access to a short-barreled AR-pattern pistol in an urban environment? The witnesses who just happened to find themselves near a strip mall’s recruiting center in Chattanooga, Tennessee, where a shooter opened fired on July 16, 2015, hadn’t considered it either. With no armed citizen intervening, after shooting into the recruiting office the terrorist managed to drive 7 miles to a U.S. Navy Reserve center where the bloodshed continued. Four Marines and a sailor were murdered, and a Marine recruiter and police officer were wounded before the shooter was killed by responding police in another gunfight.


Beliefs concerning self-defense aside, carrying a short-barreled AR-pattern pistol is legal as of this writing in many areas throughout the United States.

Meet the RUK-9BT

Rock River Arms introduced its BT-9 carbine just prior to the 2019 NRA Convention. In the popular interest of shooters who enjoy shooting pistol-caliber carbines (PCC), the BT-9 was well timed and has proven effective as a blowback-operated AR platform.

Rock River Arms
Unlike many AR-15s fitted with a pistol-magazine adapter, Rock River Arms specifically developed a machined billet lower receiver to function as a pistol-caliber firearm. Therefore, magazines lock into place perfectly. The lower also integrates an enlarged triggerguard with beveled edges that eliminates the need for three additional parts.

That same blowback action now appears in the new RUK-9BT. It has been tuned and carries a shorter 4½-inch barrel while still using the standard 9mm bolt. Worth noting to those just needing a new shorty upper for an old 9mm lower is that the bolt is contoured to fire from either single- or double-feed magazines. A lot of 9mm ARs used to be made for use with Colt-pattern magazines, which has a different feed lip arrangement and follower than the increasingly popular use of feeding from Glock-pattern magazines. So, it’s praiseworthy to note that the RUK-9BT upper will run using either type of lowers. Of course, the complete RUK-9BT features a machined billet-aluminum lower receiver that feeds from Glock-pattern magazines. Because of this, the RUK-9BT also features a last-round hold-open feature triggered by the magazine’s Glock-style follower.

Rock River Arms
The RUK-9BT receivers are machined from billet aluminum to offer unique characteristics necessary to shoot a pistol-caliber cartridge. Inside is a standard RRA 9mm bolt that can be protected by closing the shortened ejection-port cover. The magazine release was designed as a lever to engage Glock-pattern magazines but positioned so that the button is at its typical location.

With the purchase of a RUK-9BT, Rock River Arms includes one clear-body ETS 17-round magazine. If you were to buy the RUK-9BT, let me suggest that you go to www.etsgroup.us and consider getting several spare magazines. They have them available in blue and orange transparent colors as well as clear, and in capacities ranging from 10 rounds to 40 rounds for less than $30.


Rock River Arms
The RUK-9BT feeds from Glock-pattern magazines of various capacities. It ships with one ETS 17-round magazine that features a transparent body for a quick visual assessment of how many rounds remain.

The lower’s magazine well is cut to feed from almost all descriptions of Glock-pattern 9mm magazines. This is great news for those of us who own or carry a Glock 17/19/26/34 model and have stashed away extra magazines. The ability to reload either gun from the same proven-design mag is always an advantage. It’s also worth mentioning that the RUK-9BT will also reliably feed from the Glock 33-round factory magazine, which I suggest keeping in the pack. Why does someone need more rounds for self-defense than a standard capacity magazine offers? Because it sucks to have to reload when you’re taking fire. In a gunfight, people tend to shoot more rounds than they realize.

Rock River Arms
The RUK-9BT’s magazine well is perfectly designed to accept Glock-pattern mags. The angled edges act like a funnel and help to speed up reloads no matter the approach.

The billet lower is also designed with a large, ambidextrous magazine-release button that’s positioned in the familiar location for an AR, despite the Glock-pattern’s different magazine catch slot location. The remainder of the controls are one-sided for right-handed shooters.

The triggerguard is large and integral to the receiver design, providing unimpeded access to the two-stage Rock River Arms trigger. My trigger-pull gauge measured an average weight of 5 pounds, 1 ounce from 10 pulls of the RUK-9BT ’s.


Rock River Arms
With a red dot mounted and a 17-round magazine inserted, the RUK-9BT offers lightweight handling and practical performance to at least 50 yards with quality 9mm ammunition.

The upper receiver had to be machined from extruded aluminum because of the unique bolt assembly necessary for blowback operation. Unlike most AR-pattern rifles, the RUK-9BT lacks a forward assist that could be used to push a rotating bolt forward or brass deflector. Because the RUK-9BTs ejection port was reduced in size, a modified ejection port door was installed.

Rock River Arms
Tucked underneath the RRA 4-inch free-float aluminum handguard is a 4½-inch Wilson barrel complete with a Vortex flash hider. The vents help to cool the RUK-15’s gas block and three M-Lok slots allow for attaching accessories such as a compact light or handstop.

Up front is an abbreviated-length, free-float aluminum handguard measuring 4¼ inches. It not only offering vents for circulating air, it also provides three M-Lok slots for attaching accessories at 3, 6 and 9 o’clock. (I recommend a compact light and a handstop.) The handguard is indexed to the upper receiver and aligns the handguard’s top rail with the upper receiver’s. The handguard is attached to a barrel nut by two clamping screws and two locking screws that secures it from below. The handguard completely shrouds the Wilson barrel’s length except for the legendary Smith Vortex flash-hider affixed to its muzzle.

Rock River Arms Mnii
Smith Enterprises’ (SEI) legendary Vortex flash hider for 5.56 NATO-chambered barrel features a helical grooves. Though SEI offers its Vortex flash hiders with or without the grooves, here they are used to distinguish the RUK-9BT’s 9mm barrel from the RUK-15’s 5.56 barrel.

If the handguard is one of three points of control, the second and third would include the Hogue OverMolded pistol grip given the RRA-crosshair logo. Hogue’s rubber grip is the familiar standard complete with finger grooves. The third point of contact offered is the SB Tactical SBA3 Pistol Stabilizing Brace. The brace was designed for all platforms accepting of a military-spec carbine receiver extension and is five-position adjustable. The adjustable nylon strap can be used to secure the brace to one’s forearm and a sling socket enables use with a sling. This brace is ATF compliant and was designed by a U.S. veteran.

Rock River Arms
The SBA3 pistol brace from SB Tactical attaches to mil-spec carbine receiver extensions and is five-position adjustable. It is a minimalist design that enhances the effectiveness of any AR pistol, especially for shooters who can benefit from its forearm stabilization-strap system.
Rock River Arms RUK-9BT
  • Type: Blowback operated, semiautomatic
  • Cartridge: 9mm
  • Capacity: 17+1 rds.
  • Barrel: Wilson 4.5 in., 1:10-in. twist, chrome-lined chrome moly
  • Barrel Finish: Magnesium phosphate
  • Overall Length: 24 in. (extended); 21.25 in. (collapsed)
  • Weight: 5 lbs.
  • Grip: Hogue OverMold, standard
  • Brace: SB Tactical SBA3, 5-position adj.
  • Handguard: RRA Lightweight, 4.25 in., M-Lok
  • Trigger: RRA, two stage; 5 lbs., 1 oz. (tested)
  • Safety: RRA Star selector, two-position
  • Sights: RRA NSP flip-up back-up
  • MSRP: $1,350
  • Manufacturer: Rock River Arms, 866-980-7625, rockriverarms.com 

RUK-15

Rock River Arms
The RUK-15 upper and lower receiver are standard AR-pattern aluminum forgings. Controls are predictably located, but RRA Star selector lever, triggerguard and two-stage trigger would be considered upgrades to most AR-15s.

The RUK-15 is a similarly sized pistol chambered for the 5.56 NATO round. It begins with Rock River Arms' LAR-15 receivers. The upper and lower receiver are conventional AR-patterns meaning that the forged-aluminum upper includes a forward assist, pronounced brass deflector and full-length ejection port and cover. Inside is a standard bolt made of Carpenter 158 that reciprocates within a heavier full-auto bolt carrier made of 8620 steel. Because it is gas operated, the barrel is tapped directly for gas to unlock the bolt and cycle the bolt carrier assembly by means of a short, albeit straight, gas tube above the barrel. Rock River’s approach was to custom design a short, straight gas tube for this application. This also kept the steel gas block low profile so that it would fit under the same handguard seen on the RUK-9BT, which doesn’t need a gas block. 

Rock River Arms
The RRA lightweight aluminum handguard is secured to the barrel nut by two screws that clamp and two screws that thread into the nut. The result is a rigid yet straight alignment to the upper receiver that won’t twist under hard use.

Because the RUK-15 sports a barrel with a smaller-diameter bore, it may go without saying that it can’t use the same muzzle device as the RUK-9BT. The different bore sizes between muzzle devices can be instantly recognized by the serrations appearing around the 5.56mm Vortex flash hider.

Aside from the RUK-9BT’s unique upper and lower receiver assemblies, both the RUK-15 and RUK-9BT share similar components including the SB Tactical SBA3 Pistol Stabilizing Brace and Hogue grip, as well as a set of lightweight flip-up back-up sights (BUIS). If you shoot a lot, it doesn’t take long to see how lightweight mini red dot would pair perfectly with either setup.

Rock River Arms
The RUK-15 in 5.56 is gas operated, so Rock River Arms had to design a unique straight gas-tube assembly that didn’t come into contact with the barrel nut. Much of the proprietary knowledge used by engineers to make the RUK-15 reliable with such a short barrel surrounds this setup and gas port size.
Rock River Arms RUK-15
  • Type: Direct impingement, gas operated, semiautomatic
  • Cartridge: 5.56 NATO
  • Capacity: 30+1 rds.
  • Barrel: Wilson 4.5 in., 1:7-in. twist, chrome-lined chrome moly
  • Barrel Finish: Magnesium phosphate
  • Overall Length: 24 in. (extended); 21.25 in. (collapsed)
  • Weight: 4 lbs., 9 oz.
  • Grip: Hogue OverMold, standard
  • Brace: SB Tactical SBA3, 5-position adj.
  • Handguard: RRA Lightweight, 4.25 in., M-Lok
  • Trigger: RRA, two stage; 4 lbs., 3.9 oz. (tested)
  • Safety: RRA Star selector, two-position
  • Sights: RRA NSP flip-up back-up
  • MSRP: $1,040
  • Manufacturer: Rock River Arms, 866-980-7625, rockriverarms.com

At The Range

Any experienced user of SBRs knows that reliability is the primary concern when shooting short-barreled anything. The 9mm is obviously a low-pressure round. SAAMI sets a manufacturer’s limits at 35,000 psi for standard cartridges and 38,500 psi for +P loads. Because 9mm loads are developed with fast-burning pistol powders, the chambering is right at home in Rock River’s RUK-9BT with 4½-inch barrel. The only way to fully evaluate the extreme capabilities regarding the action’s reliability is to test it on a full-auto lower, which is what I did. Because the bolt in the RUK-9BT is standard, it is cut to run on a select-fire 9mm AR lower. I borrowed a sample for use at Rock River Arms’ test range and verified that it will run the variety of Glock-pattern magazines and capacities without fail.

There is a broad range of bullet grain weights available for the 9mm. For this evaluation, I tested the RUK-9BT with NovX 65-grain ARX lead-free bullet that race out of the muzzle at rifle velocities exceeding 1,800 feet per second (fps). I also evaluated function and accuracy with the heaviest 147-grain projectiles, and everything in between. Interesting to note, on a full-auto 9mm lower, the RUK-9BT fired through a 17-round magazine in 2.35 seconds and a 33-rounder in 4.03 seconds according to my PACT shot timer.

Though there were no failures in reliability, and shooters can expect a reasonable improvement in their accuracy out to 50 yards using this AR pistol versus a typical 9mm sidearm. Accuracy is where the barrel’s rate of twist should factor in your expectation, however. Typical supersonic loads including 115-, 124- and 135-grain bullet weights need progressively slower rates of twist to stabilize the bullet optimally for shooting tight groups if punching small clusters on targets is your priority. Barrels optimized for 115-grain bullets often feature a twist of 1:32. A 124-grain bullet is stable with either a 1:16- or 1:18-inch twist. An accurate barrel shooting subsonic 147-grain bullets should feature a faster twist rate of 1:10. Armed with this knowledge I’ve gleaned from others through the years, I observed that the RUK-9BT was designed for 147-grain performance, which also means it would run great with a suppressor attached. If you remove the Vortex flash hider, you can suppress the RUK-9BT using a .30-caliber suppressor with ½-36 threads.

The RUK-15 is extra special because it is a challenge to get a pistol-length barrel to run an AR reliably. The secret sauce is in the port size and its relationship to the gas block and tube. Rock River Arms wouldn’t reveal what port size they settled on, but they did admit that the magic of this pistol’s extraordinary reliability was in the port’s engineering. Not surprised, on inspection I also found a heavy H3-weight buffer slowing the cycle of operation down within the buffer tube.

Rock River Arms
Both the RUK-9BT and the RUK-15 come equipped with Rock River Arms’ polymer NSP flip-up post-aperture sights. Together, they are fully adjustable and could run as a low-profile primary system. However, the A4-style upper receiver is ready to accept lightweight miniature red dots.

Both the RUK-9BT and the RUK-15 were capable of producing several sub-2-inch loads from a bench at 50 yards. This opened up to about 6 inches at 100 yards when exploring different grain weights. I’m reluctant to emphasize either’s accuracy too much given that Rock River Arms is still experimenting with barrel combinations to maximize performance from both guns, but I was pleased to observe during a recent visit to the factory that someone behind the scenes is always working to improve their existing products. To that end, these micro ARs are certainly both capable of providing the level of accuracy to enhance our self-protection and provide some peace of mind in this crazy world that we call home.

Learn more about the RUK-9BT and the RUK-15 by visiting www.rockriverarms.com.

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