The Kel-Tec RDB

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Ever since the technology to manufacture compact semiauto rifles was developed, forward-thinking designers began to prototype such firearms with forward-placed controls. These so-called "bullpup" rifles place the magazine behind the main grip and moves the breech to the rear, usually a cartridge length in front of the butt plate.

When fitted with a 16" or 18" barrel, the overall length is substantially reduced from that of a comparablychambered semi-auto of conventional layout, naturally appealing to paratroopers, armor crews, and personnel who engage in close quarters battle. The first really successful modern bullpup was the Steyr AUG, made famous in more than a few Hollywood fiction creations.

The French followed with the FAMAS bullpup and the British replaced their version of the FAL, the L1A1, with the Enfield SA80 which evolved into the current BAE Systems’ L85A2. Relatively lately the Israelis brought out their Tavor in a few chamberings and FN the P90 and F2000. The AUG clone Microtech MSAR was introduced in 2007 but Steyr opening a US facility meant that the semi-auto version of the AUG was more readily available and it killed demand for the MSAR, which closed its doors in 2015.

There have been many more bullpups, both rifle and shotgun, but the subject of this article is the third bullpup offering from Kel-Tec CNC, the RDB. The Kel-Tec RDB (Rifle, Downward-ejection, Bullpup) is chambered in 5.56×45mm NATO and Top: Kel-Tec RDB Bullpup shown from the left side and fitted with a 20round USGI aluminum magazine. The folded charging handle is visible directly below the front sight and above the upper edge of the handguard.

The 30-round magazine’s longer projecting length does not interfere with the strong hand grasping the pistol grip. followed Kel-Tec’s 7.62 NATOchambered RFB and the 12 gauge KSG shotgun.

Like the AR-15, the RDB uses a similar multi-lugged rotating bolt with not one but two spring-loaded ejectors and with the extractor at the 5 o’clock to 7 o’clock position on the bolt face, which causes cartridge cases to be ejected downward through a sheet metal-lined central port in the RDB behind the magazine well and in front of the butt plate.

There is no ejection port on the right side of the receiver, so the RDB can be shouldered left or right without a thought to the empties flying close by one’s face, although in any event if the shooter is wearing a lightcolored shirt, expect to use some stain-removing laundry aids as the sooty cases dribble down the front of the shooter’s torso. The RDB features a reversible charging handle, which is shipped from the factory fitted to the left side of the weapon.

Partial disassembly (covered later) allows quick "mirror-image" repositioning of the charging handle to the right side of the RDB, for left-handers or those right-handers who prefer the "AK" charging handle setup of holding the pistol grip in the right

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From right to left, trigger, safety at the top of the pistol grip, magazine release in front of the magazine well, and the bolt release directly above the rear end of the magazine release. hand and tilting the RDB counter-clockwise 90 degrees to allow easy access of the weak hand to the charging handle. Kel-Tec designers thoughtfully provided a charging handle hold back notch similar to that on Heckler & Koch rifles and submachine guns, but on both sides of the RDB.

Simply pull back the charging handle fully rearward and The charging handle is reversible in a matter of seconds from the factorybuilt left side position to the right side. Below center: Charging handle fully withdrawn to the rear by the left hand. The rear sight is an allmetal Magpul MBUS Pro. The handle can be "sling shotted" or slapped down from this position to release the bolt assembly forward. rotate it upward to engage the retention notch to hold the bolt open.

The rotating bolt may be AR-15like but the bolt is housed in an AK-like bolt carrier block that rides on guide rails welded to the inside of the sheet metal receiver. The carrier block is also welded to along operating rod similar to the gas piston/carrier extension of the AK- 47/AKM but it is hollow at the rear and houses the long action spring and action spring rod. When the RDB is assembled, there arof the action spring impinges against the inside surface of the receiver which forms the butt plate.

The front of the action spring rod mates in a spring-loaded gas piston housed within the gas block near the front of the handguard. The forward tip of the action spring rod thus retains the gas piston itself inside the gas block; the gas piston is a friction fit dependent on the spring force of the three sealing rings surrounding the gas piston body and in contact with the inside diameter of the gas block.

The RDB has an adjustable gas feature consisting of a knurled knob in front of the gas block, fitted with recesses into which the bullet of a cartridge can be inserted to facilitate adjustment when the rifle is hot. Alternatively, wear gloves when adjusting the gas on a recently-fired RDB. Rotating the knob full clockwise when viewed from the muzzle conducts full gas from the barrel port to the gas piston. Most RDBs cycle with the

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In normal use the inside of the weak hand trigger finger squeezes the magazine release backward while the thumb grasps the back wall of the magazine to facilitate removal as not all magazine types drop free. The bolt lugs are visible at the right end of the magazine well itself. gas regulator adjusted three to five clicks counter-clockwise from the full clockwise stop position. There are no reference numbers or markings so one must count the clicks.

If insufficient gas is conducted to the piston, the action will short stroke, resulting in a partial extraction and possible new cartridge feed jam. The jam must be cleared one of two ways: Either through the magazine well with the bolt retracted and held to the rear by the charging handle in its notch or by disassembly of the weapon.

It is NOT recommended to fully cycle the charging handle to clear the jam with a loaded magazine in place as doing so may strip even more cartridges from the magazine resulting in a tighter jam. Again, safe the weapon, ensure the muzzle is pointing downrange, retract the charging handle, and fish out any live cartridges and the empty casing that failed to extract/eject using a screwdriver or two like chopsticks or a set of long needlenosed pliers.

The firing and operational controls are located on the pistol grip or on the receiver convenient to the pistol grip. The ambidextrous safety selector is readily activated by the strong hand thumb. The trigger is ergonomically located in the pistol grip. The magazine release is a U-shaped spring steel stamping located centrally behind the pistol grip and directly in front of the forward edge of the magazine well.

The magazine release slides in guide grooves on either side of the polymer The recess in the knurled adjustment knob is for the point of a cartridge tip for gas adjustment when the weapon is thermally hot. magazine well. Pushing the magazine release tab rearward causes the magazine catch tab to "cam" out of the magazine retaining slot and the magazine itself. The spring steel properties of the magazine release reset it to its forward position upon release.

The RDB accepts standard "NATO Draft Standard Agreement (STANAG) 4179" pattern 5.56x45mm magazines, including the ubiquitous USGI 20 and 30round aluminum-bodied M16/M4 magazines as well as the variety of commercial metal and polymer magazines. The bolt release is also ambidextrous with serrated, tapered rectangular thumbpieces on both sides of the magazine well. Pressing one of these thumbpieces downward releases the bolt.

There are four assembly groups to an RDB: Barrel, Bolt, Receiver, and Grip as Kel-Tec calls them. The Barrel group consists of the barrel, gas block, charging handle, optic rail, and polymer fore end halves. The Bolt group consists of the bolt carrier, bolt, action spring, and ac-

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The "stirrup" at the upper right is the magazine catch that engages the rectangular recess in the side of a NATO STANAG-type magazine. The left and right handguards end in short 90-degree flanges to each side, providing finger stops to keep the weak hand from sliding off the front into harm’s way. tion spring rod. The Receiver group consists of the receiver itself, a Usection heat treated sheet metal channel with four lugs into which holes are punched to accept takedown pins.

Other Receiver group components include the heat shield which serves as a cheekpiece, the stock (butt plate), and the stock cover (butt plate pad). The Grip group contains the two halves of the polymer pistol grip/magazine well assembly which contains the The rear MBUS Pro is foldable to reduce profile in the event an optic is fitted and features a folding small aperture for distance shooting.

The barrel group is readily removed forward and completely out of the receiver assembly group. trigger and other fire controls, safety, magazine well, magazine catch, bolt release, ejection port, etc. There are four captive push pins which hold the entire firearm assembled. To disassemble, first cock the weapon.

Pressing the two middle pins from left to right using either a cartridge tip or a brass punch allows the Grip group to be rotated downward slightly and permits the Barrel group to be withdrawn forward, after which the Bolt group can be withdrawn from the rear of the Barrel group. Disassembly is "assisted" somewhat by the compressed action spring which tends to push the Barrel and Bolt groups forward with the two center assembly pins removed and the Grip group tilted downward.

Removal of the rearmost assembly pin separates the rear of the Grip group from the rear of the Receiver group. Removal of the forward-most assembly pin separates the front handguard assembly from the Barrel group. To field strip the Bolt group, drive out the firing pin retaining pin from left to right using a punch, capturing the O-ring used by Kel- Tec to keep the firing pin retaining pin in place.

Withdraw the firing pin to the rear and then remove the bolt cam pin from its cam slot in the left side of the bolt carrier. Then withdraw the bolt from the carrier toward the front. The RDB’s bolt is a modification of an AR bolt in principle.

There are multiple lugs surrounding the bolt head from approximately 8 o’clock to 4 o’clock when looking at the bolt face, with the extractor located from 5 o’clock to 7 o’clock, and two spring-loaded ejector pins at 11 o’clock and 1 o’clock with the firing pin port in the center of the bolt face. The arrangement of the

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The pushpins engage the holes in the lugs of the sheet metal receiver. The fire control parts and dual mainsprings are somewhat hidden beneath the right and left halves of the Zytel polymer pistol grip halves and tubular steel compression limiters.

This makes for disassembly and reassembly operations requiring "six to eight hands" or a purpose-built assembly jig that is cost-prohibitive for the average gunsmith. features just described makes the bolt head/bolt face remind this author of a "skullface." Further stripping of the bolt is similar to that of an AR bolt, removing pins which free the ejectors and the extractor claw. There are two versions of the RDB’s action spring assembly.

The Talk about "long-throw" hammers—this hammer has the longest pivot radius of any firearm the author has examined in nearly 50 years of the study of firearm design. early type is captured inside the bolt carrier and released by depressing a small latch at the rear of the carrier. Use care and apply a restraining force to control the spring as it is freed from the carrier.

The later version of the action spring assembly is not captured inside the bolt carrier and disassem- Kel-Tec does not recommend further disassembly, and having examined the RDB after its disassembly into the four groups, I don’t either. Kel-Tec is known for its CNC machining methods and unconventional designs that make widespread use of much polymer and many, many pins, screws, and nuts.

Examining the Grip group, it is obvious that the factory makes use of an elaborate assembly jig as well as likely automated assembly. Attempting to fully disassemble and reassemble an RDB is simply outside the scope of the average gunsmith’s capabilities and there is no economic justification for making the fixturing for anyone who would infrequently see one of these firearms.

For those interested in more detail, a copy of the RDB’s owner’s manual with several "exploded views" of its components is available for download from the Kel-Tec site at bit.ly/2v Kxjx Q. Recently I was asked to look at a fairly new RDB that was intermittently failing to go into battery and the bolt carrier was "hanging up" as the bolt face contacted the barrel extension.

Since a watched pot bly of the RDB results in the action spring pushing the action spring rod well out of the carrier tube until the action spring full extended, perhaps a foot or more out of the carrier. never boils, when the owner and I examined the RDB together and handcycled the action a number of times there was no indication of a problem. We took the rifle to the range and following a magazine full of 55 grain Fed-

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eral 5.56 ammo, the bolt did fail to go into battery when released. Because the RDB’s design is so enclosed, it was difficult to see up through the magazine well but it was obviously that the bolt came to rest too far to the rear as the bolt lugs were visible. Repeated racking of the operating handle either caused the jam or cleared the jam. It was time to do some exploratory surgery. I disassembled the RDB and tried fitting the Bolt and Barrel groups together by hand.

It was obvious that the bolt carrier needed to be perfectly aligned with the barrel and looking at the inside of the receiver the bolt carrier guide rails could be seen.

I used the receiver as a guide for the carrier, a function it normally does in an assembled RDB, and brought the Barrel group together with the Bolt group without the action spring or action spring rod in order that the Bolt group could be slid to and fro as needed. ), exhibiting the extractor at the 6 o’clock position so that empties are tilted downward for ejection out the bottom of the RDB. Despite the different configuration of individual components, the RDB’s bolt is similar in concept to that of an AR.

It became apparent that there was considerable play between the bolt carrier and the guide rails inside the receiver, approximately 0.090" total as measured by calibrated digital caliper. So much so that the bolt carrier could be tilted from side to side ("yawed") and in doing so, the front faces of the bolt lugs would be offset from the gaps between the lugs in the barrel extension and the bolt face would stop against the rear face of the barrel extension lugs.

In order to prove the theory of too much clearance between the guide rails being the cause of the problem, I called a friend whom I knew to have an RDB and asked to take a look at it and get confirming measurements. In the meantime, as an experiment I placed the receiver of the problematic RDB between the padded jaws of a small bench vise and applied some clamping force to the receiver so as to force the guide rails closer to each other in the area where the bolt carrier would be with the bolt in battery.

Numerous repeated cycles of the bolt carrier resulted in no more "hang ups" of the bolt face on the barrel extension face with the receiver so "pinched" and the receiver assembled to the barrel group.

Knowing the receiver to be heat treated just from the audible "ring" it made when bumped against another metal object, and also knowing that the owner had recently purchased the RDB new, I advised him to contact Kel-Tec, explain the problem, and email them a copy of the sales receipt and take advantage of the lifetime warranty to the original purchaser. They would email in turn a prepaid shipping label and he would merely need to box it up, contact a shipping carrier driver, and wait.

Just before shipping the problematic RDB to Kel-Tec, my friend brought over his RDB and a quick examination through the magazine well showed that there was significantly less clearance between the bolt carrier guide rails in the receiver in his RDB than in the customer’s. Disassembly of both bullpups and measurement of the rail spacing in the receivers yielded the shocking result that the rails of the RDB with the bolt hang up issue were 0.040" widerspaced than those of RDB from my friend.

There was simply too much "slop" between those rails to accurately guide the carrier and bolt into battery with 100% certainty. The wait for Kel-Tec’s turnaround for the RDF’s repair turned

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Far The piston is retained in the gas block by the front of the action spring rod. out to be about seven weeks and the owner anxiously brought it over for examination. When we unpacked the rifle, the receiver had the same serial number as before. Prior to shipment we had applied a small ID mark to the interior of the receiver and that telltale was there. It was the original receiver, but what of the guide rail spacing?

The rails turned out to be closer together but only in the area where the bolt carrier would be with the bolt entering battery / engaged into battery. The guide rail spacing near the butt was still approximately 0.040" wider than that of the RDB owned by my friend but only 0.009" wider at the critical spot where the carrier would be as the bolt entered battery. The bolt carrier guide rails inside the receiver were 0.040" farther apart than the guide rails on a second, problem-free RDB measured by the author.

We took turns repeatedly cycling the bolt carrier manually, probably 300-400 times the evening of the RDB’s return from the Florida factory. Not a single hang up occurred. The proof of the pudding being in the eating (to correctly state the adage), as soon as he could the owner took the "factory-adjusted" RDB to the range and called me reporting that in six 30-round magazines fired there were no hang ups or stoppages of any sort.

While I believe it would have been possible to use a padded vise to permanently "spring" the sides of the receiver closer together to reduce or eliminate the excessive gap between the bolt carrier guide rails, controlling the necessary amount of plastic deformation of the receiver may have been problematic. Squeezing the sides of the receiver sufficiently to get the metal to "move" and not spring back could very well have resulted in the guide rails being too close, requiring respreading of the receiver.

Tempering back the receiver, adjusting the spread of the guide rails, and retempering to the original hardness would have been the technically correct remedy but that would likely have required refinishing and may have affected the appearance and readability of the etched serial number. If the RDB had been purchased secondhand, the factory warranty would have been void. I would have explained the planned repairs to the owner and with his sign-off as to understanding the risks, proceeded with pinching the receiver.

However, given the Kel-Tec lifetime warranty to original owners, and it being anew firearm, I’ll stand by my recommendation to the owner to ship it back to the manufacturer. In this case it was best for all parties for the problematic RDB to go back to the factory. AG