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Gone: the ultimate PDW caliber? Who killed the 6x35mm TSWG?


For some reason it seems like the older you get the faster time passes. Recently while looking through some old files I was surprised to notice ten years have passed since I first wrote about the 6x35mm TSWG (Technical Support Working Group) cartridge and the compact PDW that chambered it. At the time of its introduction the 6x3 5mm TSWG appeared to be a well thought out and logical solution to the personal defense weapon (PDW) conundrum. Even more so when you consider the ultra-compact and handy 4.3-pound short-barrel carbine developed to fire it. Ten years down the road though and relatively few have heard of this cartridge. So who murdered the 6x3 5mm TSWG?

For those of you unfamiliar with the cartridge, let's turn back the clock to August 2006 and revisit its history. To do so we have to travel to Titusville, Fla., the home of Knight's Armament Company or KAC as they are often called. KAC has been performing research, development, testing and evaluation of new small arms and ammunition for the U.S. Government for more than 30 years. Founded by C. Reed Knight, Jr. the company began supporting the U.S. military by performing work for small, specialized units. As an example, they rebuilt Stoner 63 machine guns for the U.S. Navy SEAL Teams, developed a family of sound suppressed revolver rifles for the Special Forces and designed/manufactured over 5,000 "snap on" sound suppressors for the then new M9 service pistol.

During the invasions of Panama and Grenada, Knight's Armament observed a need for the ability to mount mission-specific accessories onto the M16 family of weapons. This led to the development of their MIL-STD 1913 rail systems. Other products include rail covers, receiver plate sling mounts, back up iron sights, all manner of optical sight mounts, full-auto two-stage match triggers, sound suppressors, flash suppressors for .50 caliber machine guns and much more. Not to be forgotten is their resurrection of Stoner's original 7.62x51mm NATO AR-10 rifle into a viable modem sniper system. Whether you love them or hate them, you have to admit KAC laid the foundation for the modern evolution of the AR platform.

The 6x35mm TSWG cartridge was developed with a specific and specialized purpose in mind. It was not intended to be a general-purpose cartridge like the 5.56x45mm NATO. Rather it was intended to be a better solution for use in short barrel PDWs. Keep in mind, at this time there was still a great deal of interest around the world in a small and lightweight cartridge/weapon combination capable of both penetrating soft body armor and providing acceptable terminal performance. The genesis of this concept originated when NATO began to look for a modern replacement for their aging 9mm service pistols and submachine guns. This led directly to the development of both Fabrique Nationale's 5.7x28mm cartridge/weapons family and Heckler & Koch's 4.6x30mm MP7 PDW.

While both FN's 5.7x28mm and HK's 4.6x30mm are interesting cartridges, their terminal performance has been questioned by many well-respected authorities since their inception. KAC recognized the shortcomings of these two cartridges in this regard and specifically developed a new round with more punch. As an example, HK's 4.6x30mm MP7 drives a 24.7-grain 4.6mm copper-plated solid steel slug at approximately 2,379 fps. In doing so it generates 312 foot-pounds of energy and is capable of penetrating a NATO CRISAT target at 200 meters. HK's MP7's arch nemesis, FN's 5.7x28mm P90 PDW, on the other hand drives a 32-grain .224-inch diameter steel-core slug at 2,350 fps. This SS190 load generates 390 foot-pounds of energy and is also capable of defeating a NATO CRISAT target at 200 meters. While both of these loads are capable of penetrating soft body armor, their ultra-light, small caliber AP projectile's performance in gelatin testing is fairly unimpressive.




KAC's take on a PDW cartridge was a bit different. The 6x35mm TSWG drives a slug not only larger in diameter and heavier than both the 4.6x30mm and 5.7x28mm, but also our standard 5.56x45mm M855 ball round. Loaded into a 35mm long boxer-primed brass case is a 65-grain .243-inch diameter Open Tip Match projectile developed by the engineers at Hornady. When fired from the 10-inch barrel of KAC's Personal Defense Weapon, this load is reported to have a muzzle velocity of 2,425 fps. At this speed the 65-grain slug generates a respectable 831 foot-pounds of energy and is capable of penetrating soft body armor at 300 meters. Obviously KAC's 6x35mm TSWG offers a substantial step-up in performance compared to the two small European PDW rounds. Plus, it's a huge leap up in performance compared to a traditional 9x19mm submachine gun.

The cartridge itself was optimized for use in a 10-inch barrel, and develops about the same--KAC claims slightly more--kinetic energy as a 5.56 M855 ball round when fired from a 10-inch AR. However, unlike a 5.56x45mm round, the 6x35mm is not cursed with an ear-piercing report or retina-melting flash signature. Since the round was designed for use in a 10-inch barrel rather than a 20-incher, the propellant is consumed in the barrel rather than in the atmosphere. The result is a relatively mild sound and flash signature, even when fired on full automatic.

It appears KAC developed the 6x35mm TSWG round directly from a 5.56mm round. Putting a caliper to several unfired rounds revealed a rim diameter of .375 inch and a case head diameter of .372 inch, the same as an unfired Hornady 5.56 round I compared them to. Case length of several fired rounds averaged 1.38 inches while neck length is approximately .20 inch. Overall cartridge length of several loaded rounds averaged 1.927 inches. Popping one of the little buggers open for a look inside revealed a charge of extruded smokeless powder. A standard non-corrosive boxer-type small rifle primer lights the propellant. Sitting atop the propellant charge is a .243-inch 65-grain Open Tip Match bullet with a cannelure for reliable operation in fully automatic weapons. The slug measured .913 inch long and has a boattail to enhance its exterior ballistics. The open tip design is obviously intended to improve terminal performance, but this is no sniper round. The question is, how far this design will reliably fragment considering its 2,425 fps muzzle velocity?

I like the concept of a modern personal defense weapon that's accurate, easy to control on full auto, has a relatively flat trajectory and a mild sound/flash signature. Knight's Armament Company's 6x3 5mm TSWG cartridge provides all this. During testing at Knight's facility in Titusville, Fla., I found their new compact PDW very easy to hit with on semi-automatic and quite controllable on full automatic. Even while dumping entire magazines in one long burst it was relatively simple to keep your rounds on target despite the weapon's light 4.5-pound weight. Best of all, unlike a M16 shorty or AKS-74U there was no blinding thermonuclear muzzle flash or horrendous pressure wave. You pressed the trigger, the gun rattled away and crap flew off the target. Simple. [ILLUSTRATION OMITTED]



So what happened to it? Where is the 6x35mm TSWG cartridge today? When it comes to the commercial market, the 6x35mm TSWG doesn't even exist. To be fair, neither does HK's 4.6x30mm. Both were developed for specific military applications and never successfully crossed over to the commercial side. FNH's 5.7x28mm has been more successful and, unlike the other two, has become a viable commercial cartridge with a small but happy cult following.

I'd like to point out that the U.S. commercial market has changed dramatically since 2006. AR pistols and registered short barrel rifles (SBR) have become very popular. Many more shooters are willing to do the NFA paperwork today to legally own an SBR than at any other time in the past. Sound suppressors are also continually growing in popularity. I wonder if there might be a commercial market for the 6x35mm TSWG cartridge today. While I don't see KAC offering a commercial version of their PDW any time soon, it would be possible to chamber the cartridge in other designs. The most obvious would be an AR, which KAC has even done with their SR-635. Of course the downside to this would be the expense of oddball ammunition, proprietary magazines and a new platform.


Plus, as interesting as KAC's 6x35mm TSWG cartridge is, there is another cartridge with a 35mm case length of perhaps more value, the 300 AAC Blackout. The 300 AAC Blackout (7.62x35mm or 300 BLK) is an obvious evolution of the .300/.221 Fireball Wildcat and JD Jones' .300 Whisper. The 300 BLK came about after Robert Silvers, then the Research and Development Director of Advanced Armament Corp. (AAC), was contacted by a Government customer in 2010. The customer had a requirement for this concept and requested AAC to produce firearms with Remington Defense manufacturing .300/.221 Fireball type ammunition. This request would lead directly to Silvers developing what we know today as the 300 Blackout cartridge.

The goals of the project were fairly straightforward:

1. Create a reliable .30-caliber cartridge compatible with the AR platform.

2. Create the optimal platform for sound- and flash-suppressed fire.

3. Develop supersonic ammunition which matches 7.62x39mm ballistics.

4. Provide the ability to penetrate commonly encountered barriers.

5. Utilize existing M16 magazines at their full capacity.

6. Encapsulate these capabilities in a lightweight, low recoiling yet durable package.

During a conversation I had with Silvers he commented, "300 AAC Blackout was designed to be the most powerful way to shoot .30 caliber from an AR-15 while remaining compatible with normal 30-round magazines. A fresh name was needed so that we could design the chamber with the exact specs we thought best. 300 AAC Blackout is a name both consistent with full power ammo, but yet stealthy at the same time. For 300 AAC Blackout, we went back to the official Remington .221 Fireball drawing and used the exact dimensions from the rear, while doing the front portion in an optimal way. We picked a longer throat than in some Whisper chambers--to allow a 220-grain Sierra MatchKing to be loaded to full magazine length while being 0.010-inch from the rifling. That keeps pressure down, and allows for full power ammo to be loaded hotter for more velocity." The 300 BLK was designed from the ground up to utilize standard 5.56x45mm AR-15 bolts and magazines. Plus, the cartridge and chamber dimensions were specifically optimized for use with both supersonic and subsonic ammunition. Sound suppressors continue to grow in popularity in the U.S. Hearing loss is a real concern for many shooters, your author included. Sound suppressors make shooting much more pleasant and enjoyable. With the 300 BLK you can enjoy running both supersonic and subsonic ammunition through a sound suppressor. Not only that, but the cartridge works extremely well in ultra-short barrels. So if you don't mind putting the paperwork through, you can have an ultra-short and suppressed 300 BLK AR carbine. What about if you do mind putting paperwork through? Is there still any reason to consider the 300 BLK? Absolutely! The vast majority of American shooters and hunters will select 16-plus-inch barrels. The 300 BLK is right at home in lengths like this and, properly built, will allow you to seamlessly switch between supersonic and subsonic ammunition.

I will readily say comparing the 6x35mm TSWG to 300 AAC Blackout is a bit like comparing apples to oranges. Yes, they were both initially developed for military projects. But they were intended for different tasks, although there is definite crossover. In the end the 300 AAC Blackout made that transition from military to commercial sales and has become extremely popular. The KAC 6x35mm TSWG did not. However it remains an interesting cartridge with a lot of potential. As much as it intrigues me though I doubt we will see it resurrected any time soon for commercial sales.


The unfortunate reality is, few reading this will ever have the chance to even see a KAC 6x35mm TSWG PDW. This is a shame as it is a fantastically fun little piece to shoot. It is one piece I'd love to have in my gun rack. So I thought I'd share what KAC's PDW is like on the range. It's a good-looking piece that grabs your eye. It is both familiar looking yet different at the same time. At just 4.3 pounds with an 8-inch barrel, it's a very light package. With the stock extended it feels quite comfortable, and the pistolgrip fit my hand well. The controls are well laid out, and the weapon is easy to operate from either shoulder.

To start the fun insert a magazine straight up into the mag well until it locks in place. Then hit the bolt release and thumb the selector to semi. The controls are where you'd expect them to be and are easy to manipulate. Pressing the trigger produces a mild report, milder recoil and a 6mm hole in whatever you are pointing at In this regard it is very pleasant to shoot. Not only that but it is very easy to control when firing rapid shots on semi-auto. When you have a feel for the piece thumb the selector to auto and grab some trigger. The PDW growls to life and spits brass at what I would guestimate to be 750 to 800 rpm. Again, recoil is mild, the weapon is very easy to hit with and quite controllable even during long bursts.

At just 28 inches long with the stock extended, the little devil handles well and is comfortable in the hand. Magazines drop free with the push of a button, and lock easily into place. Rounds load smoothly, no muss no fuss. The charging, handle retracts easily and the selector and bolt release are simple to manipulate. The stock locks rigidly into place with zero slop and provides a good cheekweld with the iron sights. Push a button and it folds neatly to the side reducing overall length to just 19.5 inches. Accuracy? Knight's says the combination is capable of 2 MOA accuracy. I was only able to fire on a 25-yard range so cannot comment other than to say that at this distance it's capable of surgical accuracy. However, I would expect the design to shoot well within what is required from a PDW.

Knight's Armaments 6x35mm TSWG PDW is a handy little automatic weapon. It's extremely light, balances well, has excellent human engineering and easily accepts accessories. It's also very simple to operate, easy to hit with and has mild recoil. Plus, unlike a 10-inch 5.56x45mm there is no deafening report or blinding muzzle flash. It's a shame it wasn't more successful.


Knight's Armament Company


Hornady Manufacturing Company


Projectile diameter: 243 inch
Projectile type:     Open Tip Match w/cannelure
Projectile weight:   65 grains
Projectile OAL:      .913 inch
Cartridge OAL:       1.927 inches
Case length:         1.380 inches
Head diameter:       .372 inch
Rim diameter:        .375 inch
Muzzle velocity:     2,425 fps from 10 inch barrel
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Author:Fortier, David M.
Publication:Firearms News
Date:Apr 20, 2016
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