HK Blackouts – HK237, HK337 and HK437 in .300 Blackout/Whisper

At Enforce Tac this spring Heckler & Koch Germany displayed the HK237 and HK337 officially for the first time, but there were no details to be had. Now we’re teased with some more technical details.

Heckler & Koch may not like me, or you, but I am in love with the looks of the HK337. But I do like short AR15s / M4s overall, and a LWRC IC-PDW is a beautiful thing except it’s not a 300 BLK.

The HK337 could be my favorite looking HK thing since the MP7. Mount a sound suppressor and the Honey Badger PDW may not be dead after all? Typically H&K runs B&T sound suppressors on their rifles.

228 millimeters is 8.98 inches. That’s the length of the HK237 and 337 barrels. A bit longer than the original Honey Badger, and a bit less refined in some sense, but this one is making it into at least some sort of official volume production.

While the HK337 uses the HK416A5 as a base, the HK237 comes out of the G36 and the HK233 models TFB recently wrote about.

Instead of 5,56 x 45 mm, these models run in .300 Blackout / Whisper. And as you can see in the picture below, which was taken in Heckler & Koch’s booth besides the rifles, that makes quite a difference in terms of bullet weight.

You can notice that the angle of the pistol grip on the HK337 is much steeper than on other models, similar to the Magpul MOE K2 grip. The K2+ grip is my personal favorite, and works well on PDWs, but I don’t mind them on long range rifles either.

Or as Magpul themselves describe the K2 grip: “The steeper vertical grip angle is optimized for use on PDW (Personal Defense Weapon) configurations and improves comfort and control on short LOP rifles that bring the primary hand in closer to the shooter’s body.”

I think HK calls their grip the V2.

For the HK337 pictured, I think there’s room for improvement for someone looking to making their rifle even more compact. If there’s an HK416C PDW stock around, please put that on the 337 and send us a picture.

The signature JS helped translating the German in the magazine ES&T 6/2017 into English:

The *37-series are 300Whisper/300BLK variants as possible ancestors to the MP5 in situations where there’s a need for a higher ballistic power than a mp. The 237 is based on the G36, the 337 on the 416 and the 437 on the brand new 433. The system is manly based on their 5,56 brothers so you don’t need to train with the expensive Blackout ammunition, but still train one to one. The only problem till now is how to mark the parts especially mag and chamber as BLK or NATO so there won’t be confusion in rapid situations

“The HK237 is a mid-range weapon based on the G36/HK233 with the high-performance caliber .300 Blackout / Whisper.”

HK237 | Technical Data

HK237 – .300 Blackout/Whisper

Calibre .300 Blackout/Whisper
Operating principle Gas-operated
Magazine capacity 10/20/30 rounds
Modes of fire 0-1-D
Rate of fire approx. 750/min
Length min./max. approx. 710/760 mm
Width approx. 66.0 mm
Height approx. 280.0 mm
Barrel length approx. 228 mm
Weapon approx. 3,300 g


Below: This is a close as I got to the HK237 in the spring of 2017 at EnforceTac Germany.

It’s frustrating when companies put their objects of interest behind a reflective glass, onto which they beam massive amounts of light. It makes it virtually impossible to get a good photo, but with some editing in Photoshop some of the plastic and metal from HK Germany came out OK.

Note the metal vs. polymer parts. Unlike the HK433 which seems to have had several revisions of the upper and lower, the HK237 seems fixed in design. Reference to earlier TFB article on the subject HERE.


Over to the “337”:

“The HK337 is a mid-range weapon based on the HK416A5 with the high-performance caliber .300 Blackout / Whisper.”

Above: HK337 with EOTech red dot and folded back-up sights. The stock looks a bit clumsy for PDW work, but notice the pistol grip which has a steeper angle than normal.

Below: My pictures from EnforceTac, Germany 2017. Notice any changes to the design?

HK337 | Technical Data

HK337 – .300 Blackout/Whisper

Calibre .300 Blackout/Whisper
Operating principle Gas-operated
Magazine capacity 10/20/30 rounds
Modes of fire 0-1-D
Rate of fire approx. 750/min
Length min./max. approx. 710/760 mm
Width approx. 66.0 mm
Height approx. 280.0 mm
Barrel length approx. 228 mm
Weapon approx. 3,300 g


The “D” in Modes of Fire stands for “Dauerfeur.”  That’s what you and I call Fully Automatic Fire.

Below: The HK337 in Europäische Sicherheit & Technik 6/2017.

The HK337 in Europäische Sicherheit & Technik 6/2017.


EnforceTac show guns had metal magazines. Press picture has polymer.

The RUAG portfolio of ammunition for these rifles. Notice the .300 Whisper SWISS P Final edition 200 gr.

Looks mean, like a Black Hole that wants to swallow you.

For the HK437 there are no pictures or technical data available yet. All we know is that it is going to be a fully-automatic “assault rifle” in .300 Blackout / Whisper.

To remind you, this is what the HK433 in 5,56×45 NATO looks like:

We have to imagine what an HK433 with a 9″ barrel looks like until we get some official pictures, and it’s going to look rather nice I think.

The third digit , the “7” in “437” indicates the caliber, and the “7” means .300 Blackout / Whisper.

“1” means 7,62×51 NATO (ex. HK241) and “3” means 5,56×45 NATO (ex. HK433).

TFB will dig deeper into HK’s new product outline and numbering system soon.

As mentioned before, the HK line of “assault rifles” is getting confusing and this is even before they’ve added more calibers and model names. Most of the data here is HK’s own homepage and offers some help, but all the technical details are far from revealed.

Is HK pushing out model after model in an attempt to increase the company’s value? Or are they releasing new products in a reply to customer’s demands? Regardless of which their factories seem to be pushing full capacity and full shifts.

I think Heckler & Koch are setting up a new system to name their Firearms, to make it easier and more accessible for the upcoming tenders around the world.

Sweden are going to change their AK5 (FN FNC) into something else, Finland are changing or at least upgrading their Valmet RK, Germany too and the list of Nations and Special Forces just goes on and on.

ES&T 6/2017, packed with new products from Heckler & Koch Germany. Source.

HKs overview, from the German Homepage. But this is not the full picture, there are more models around.

Let’s round off the barrel length of the HK237/337 to 9″ (228 mm). To compare the HK416 A5 only comes as low as 11″ (279 mm). The HK417 A2 in 7.62 x 51 mm runs 13″ minimum. Only the G36 C3 in 5,56×45 mm can compete with the same barrel length (228 mm).

Heckler & Koch 237/337, for when you think the MP5 K PDW seems a bit old, lacks sting and should have retired long time ago.

Heckler & Koch 237/337, for when consider the MP7 PDW too small and too much sub of a submachine gun.

It will be interesting to see which countries and which special units that will show an interest in these new HK models.

Eric B

Ex-Arctic Ranger. Competitive practical shooter and hunter with an European focus. Always ready to increase my collection of modern semi-automatic firearms, optics and sound suppressors. Owning the night would be nice too.


  • Jason Culligan

    Is HK leaving the naming of their guns to drunk interns? Going through their European catalogue is like trying to solve a Sudoku puzzle.

  • Jon E

    Why do they keep insisting on using keymod?

    • randomswede

      It’s even worse, it’s not keymod, it’s “HKey interfaces”, not compatible with keymod from what I’ve read.
      The M-LOK nut is still being investigated to see if it needs to be ITAR regulated. If they decide it doesn’t need to be regulated I hope that the “NATO Accessory Rail” can be the “outie” and M-LOK kan be the “innie” standard and everything else can fade away.

      • A Guest

        I’ve heard the opposite, that quite a few (though not all) Keymod things fit on it.

        But backwards. Confusing…

  • pun&gun

    Looks like they took a page from FN’s book and gave up on trying to match shades of FDE. This one looks a lot better, though.

    • Dr. Longfellow Buchenrad

      Making your gun a “two tone tan” (or 7 tone tan) is much easier than making it all match.

      Just make all your colors different enough that they dont like they were supposed to match.

  • G

    Honey badger concept is not dead at all. The Sig MCX which is an evolution of the honey badger won the LVAW contract so is in actual use with the US military and not in a one off capacity. This HK may be too late to the party in fact instead of at the forefront.

    • Henry Reed

      Source showing its active use?

    • Dave D

      More importantly is Kevin Brittingham’s Q is currently selling the genuine Honey Badger as we speak. Honey Badger is not dead.

  • JumpIf NotZero

    Ah… anyone remember the 6.8? I haven’t seen a lot of those fanboys around… they were always yelling about how 300blk was a pointless round and just a fad.

    • Anonymoose

      6.8 was big with people who hunted deer with ARs until a couple of years ago when certain states started allowing .223 for deer. I saw a discussion about .338 Spectre (6.8SPC/10mm Magnum/.30 Remington necked up to .338 for both supersonic and subsonic use) recently, and I had thought that cartridge was dead for a while.

  • BaconLovingInfidel

    “The *37-series are 300Whisper/300BLK variants as possible ancestors to the MP5 in situations where there’s a need for a higher ballistic power than a mp.”

    JS gets an E for effort, but I think ancestors should be “descendants” or “replacements” or “alternatives”.

  • BaconLovingInfidel

    Why torture us? We’ll never see anything like this stateside.

  • iksnilol

    Isn’t advertising in the comments slightly against the rules?

  • Jared Vynn

    That 237 is ugly as sin. And not just because of that stock.

  • Some Guy

    1 should be the mgs like the Hk121/MG5 and no first digit should stand for the roller delayed guns.

  • Anonymoose

    The MP5 was originally the HK54. It’s a two digit system for their original roller-delayed designs, and then they skipped the 1xx when they developed the new “HK293” SL8 replacements that they’re selling in Europe. There was also an HK221 that was a railed-up FN MAG they tried to sell to France a few years ago. The only HK1xx I know of is the HK121/MG5.

    • randomswede

      HK293 fits with the SL8 it being based on the G36 action, 9 was supposedly civilian USA market semiauto rifle and 3 for 5.56 (4 was supposedly the original number for the German civilian market)

      But perhaps the roller locked system is considered their 0 series internally and the 1 series is the FN MAG style system?

      • Anonymoose

        The HK293 is the replacement for the SL8, though. The HK121 is not a copy of the MAG. The 221 was the railed-up MAG and the 121 is just a scaled up MG4 (which was originally called the MG43, which doesn’t fit with the usual naming scheme).

        • randomswede

          Yes, I got that discombobulated, “the HK293 and the SL8 both use G36 “based” action”.
          And I guess I got my MG actions confused, I’ll have to hit the books, again.

  • Suppressed

    Aside from the tasteless advertising that Iksni mentioned, the focus here is on piston-operated .300 blk mostly-SBR’s, not simple stuff that most of us can piece together at home on a Tuesday evening.

    • RShanger

      I don’t work for the company. I just follow them.

      • Frank

        Sounds fake but okay

  • Suppressed

    The 433/437 is the prom queen out of that bunch. Much want.

  • Dave Buck

    That HK 237has the strangest looking ‘butt stock’ (and I am being pretty charitable by calling it that) I have ever seen, bar none. And I thought that the California-compliant stocks were ugly. This wins the ugly competition hands-down. Wonder of they offer an alternative stock? I don’t care how wonderful it supposedly is, I wouldn’t give it house room in that configuration.