HK’s Other 4.6: the HK36 in 4.6 x 36

Around 1970, Heckler & Koch was doing well, but their restless engineers were thinking: what’s next? One thing we learn from history is that no weapons system lasts forever, and there was maybe one more go-around in the company’s present line of roller-locked weapons, trading some militaries’ 7.62 NATO weapons for 5.56 NATO ones. But what could offer stingy weapons procurers enough reason to stop sitting on their wallets?

HK 4.6 x 36mm, made 1971. For sale here. It seems possible that there was only one lot each of the “soft core” (lead, this) and “hard core” (tungsten carbide) FMJ.

The company explored many ideas, in two major strains. One is now well-known: caseless ammunition with a radically new action and new modes of fire, which became the G11 through many, many series of tests and evaluations in the 1970s and 1980s. The second was, perhaps, meant as a technical backstop if the G11, a technical stretch, proved infeasible. It became the HK36 — not the G36, the technical backstop HK had to create after the G11 failed, but the very obscure G36. The rifle existed in, perhaps, three prototypes. It used a unique 4.6 x 36mm intermediate cartridge.

HK 36 factory photo, as published in Full Circle. This is the configuration we call Prototype 3.

The Big Ideas: Weight and Spoonery

When we referred to this as the “other” 4.6, we’re referring, of course, to the fact that this is not 4.6 x 30 HK round used in the familiar (at least, in appearance) MP7 series widely used by US and foreign special operations forces. The 4.6 x 30 is the latest of HK’s many attempts to make an even smaller caliber round, but it was aimed at a different objective: the short-range SOF and LE submachine gun, making most shots inside 100 meters; it has very light bullets (31-40 grains for warshots) and is a hair over half the weight of 9×19 or 5.56×25 ammo, allowing a reduction in operator burden (or an increase in ammo load, naturally).

The 4.6 x 36 was developed in the 1960s to meet a different requirement entirely: that of a normal assault rifle intermediate cartridge, with engagement ranges mostly inside 300 meters. Two ideas drove the 4.6 x 36: reducing ammunition and system weight for a given effect, arguably the longest-standing trend in firearms design, and increasing terminal effect in the intended target, to wit, enemy homo sapiens. The first objective drove the reduction in caliber and length. To get to acceptable lethality, higher chamber pressures (51,200 psi CUP) were accepted, but the light projectiles (42 grain hard core/54 grain softcore) didn’t reach outlandish velocities (2,600-2,800 fps). It required a fast barrel twist to stabilize the light projectiles; 1 turn in 6.3″ was selected. HK claimed the round shot flat, allowing it to print to point of aim from 0 to 300 meters without any need for range compensation by the shooter or the sight.

The “spoonery” of the subtitle refers to an invention of Dr Gunther Voss of CETME, which remained in symbiosis with HK itself at least at the time he applied for German and US patents in 1964 and 65 (his US Patent, 3,357,357, was granted in 1967).

…to provide a rifle bullet wherein the tip of the bullet is of an asymmetric shape. When this bullet strikes the target, forces are generated which accelerate the bulet inclination.

It is stil another object of the present invention to provide a rifle bullet wherein the turning moment produced by the inclination accelerating forces increases and the bullet inclination is produced more rapidly when the distance between the bullet center of gravity and the bullet tip is greater. It is possible to increase the effect produced by the bullet tip asymmetry through the backward displacement of the bullet center of gravity.

The CG change could be produced by a dual-material cored bullet (later Russian rounds would take this approach, without using Voss’s tip).

Voss 4.6 x 36 Löffelspitz (l.) with 5.56 x 45 for comparison.

Voss further believed that by increasing terminal velocity with the subtly asymmetric bullet tip he called the Löffelspitz or “spoon tip,” he could reduce caliber without losing lethality, and without having to “underspin” the bullet, which was widely understood to be Armalite’s approach to small caliber lethality.

In addition to the effective range increase, a bullet with these characteristics offers the advantage of the possibility of reducing its caliber without decreasing the detaining power obtained with the calibers used until now.

“Detaining power” is a euphemism used throughout the patent application. But clearly, the one biggest Big Idea in the HK36 was this ammunition.

The Three Known Prototypes or Versions

It is possible that some of these are actually the same rifle before and after rework. The fairly comprehensive (to its date) HK reference The Gray Room does not include a picture of an HK 36, suggesting that this may not have been preserved by the firm (or it may not be in display condition). Full Circle only includes handout publicity pictures.

The receiver of the rifle is very slender and short and, while surviving weight figures (6.3 lb empty) generated by marketing personnel based on prototypes are hard to reconcile with real in-service weights, it should have been much lighter than other HK rifles and more competitive with AR-15 based contemporaries.

Prototype 1 had a very conventional HK roller-lock styled receiver and magazine well, and very conventional HK (as far back as CETME) drum sight. It showed a relatively early plastic HK lower marked 0-1-30 and had an unusual sliding buttstock, clearly inspired by the Colt CAR-15, even though the HK36 did not require a buffer tube.

Prototype 2 also had a fixed magazine well, but the drum sight had been replaced by an, also Colt- or Armalite-inspired, carrying handle/sight mount. A reflex sight is contained within the after third of this sight, but we’ve never seen pictures of it, or of its reticle; we do note that apart from Prototype 1 (above), all HK 36 photos appear to be innocent of any foresight or any provision for iron sights. This image was featured in the 1975 Jane’s Infantry Weapons edited by FWA Hobart. Hobart reproduced a factory brochure for the rifle inside the book. He also, at the same time, featured this firearm in an article in National Defense, the magazine of the (then) American Defense Preparedness Association (which was earlier the Ordnance Association, and would later be the National Defense Industrial Association). By this time, possibly unknown to Hobart, the HK 36 was destined for the back burner as the caseless project was beginning to look feasible.

That picture doesn’t really do the sight-tower justice. It would be preserved in the next prototype and we’ll see it from some more angles.

Prototype 3 took another turn in the direction of space age looks with a fixed stock with a high center so that the recoil thrustline is barely offset from the stock centerline. This would have the  effect of reducing muzzle rise in high-rate fire, including auto- or burst-mode fire.

The selector now has four positions: 0, 1, 25, and 3, for a three-shot burst. This appears to have been a burst at normal cyclic rate.

The unusual magwell appears also to be a little bit inspired by Armalite concepts: a disposable waffle-reinforced magazine insert made of aluminum.

Changing a magazine was a Heath Robinson task on the HK 36; it appears from surviving photos that you have to move the mag well latch to the rear which would let the spring-loaded side door open and then you could insert the 25-Round magazine insert into the well and press the side door closed. At this point you could resume fire.

It may have been even more complicated than that. This is how Major Hobart explained it in the National Defense article (via Full Circle, p. 346):

The magazine is charged as follows:

At the bottom of each side is a milled button attached to a spring-loaded chain carried inside the magazine. When the buttons are pulled down, the chain is extended and held out. This pulls down the magazine platform and compresses the magazine spring. The rear of the magazine is open, and the 30-round box is placed on top of the followers. A further pull on the chain releases the holding catch.

The magazine platform rises under the cartridges and passes inside the containing box. The chain is taken up into the magazine. The first round is now in position for loading, and when the bolt comes forward the top cartridge is fed into the chamber. The magazine is sealed against the entry of dirt, snow, etc. As subsequent rounds are fired, the magazine spring drives the follower farther up inside the ammunition box. When the last round is fired, the bolt is held open. When the chain is pulled down, the empty box is ejected, the magazine spring is fully compressed, and the platform is pulled down to allow the next ammunition pack to be inserted.

(This is what happens when you ask a room full of guys whose names terminate in Dipl. Ing. to simplify something). HK claimed that this would “reduce weight and cost.”

It’s unfair to judge the magazine system based only on images and descriptions, but the temptation to pass judgment is strong. In any event, it is not the only ergonomic question mark with these firearms. The usual HK selector switch seems to call for the usual double-jointed thumb, especially on the burst setting; also, a stock weld of any type looks practically impossible, whether you’re using the fixed or sliding stock versions. (In true HK roller-lock fashion, they’re easily interchangeable. HK was modular before modular was cool).

The close-up of Prototype 3 shows the unusual shape of the forward carrying-handle pillars, and the only reason we can think that they’re bowed out like that is to keep them out of the field of view of the mysterious reflex sight. At around this time, HK was working with Hensoldt on a reflex sight for the G11; this might be the same sight.

Note that these “Prototype numbers” are not anything assigned by HK, but something that gun watchers have applied to these photos over the years as they’ve surfaced. We’re not aware of any picture showing more than one HK 36 in any one place at any one time, so it’s quite possible that there was only one prototype, and it went through several different reconstructions. It’s also possible that at least some of the weapons in the factory photos are actually mockups or dummies, and were never built as working firearms. The existence of quantities of the 4.6 X 36 ammunition argues for the existence of functioning prototypes.

What Happened to the HK36?

We know, in broad terms, what happened with the project. As the 70s wore on and the G11 project for a 4.9 mm (later 4.7 x 21) caseless Wundergewehr came together technically, the HK 36 and its unique 4.6 x 36 mm round vanished back into the swamps of, if not Mordor, at least Oberndorf. The G11 project was all-consuming, and it was this close to Bundeswehr adoption and standardization, having demonstrated a 100% pH improvement over the G3 rifle, when it was overcome by events. The Berlin Wall crumbled, and Germany entered the phase of Wiedervereinigung – the reunification of a nation divided in twain for almost 50 years. With the defense demands that resulted from this unexpected boon, including the challenges of merging two completely incompatible sets of armed services, it would have been irresponsible to sink great resources into rifle re-armament — so they kicked that can down the road, and stuck with the obsolescent G3.

The G11, which had already been rejected by the US Army when it cancelled the Advanced Combat Rifle procurement program in 1990, went into the lockers, too, and HK was briefly without a future in the infantry rifle market (right when worldwide Police/SOF enthusiasm for its submachine guns was running out of steam).

When HK found its future again, it wouldn’t be roller-locked or caseless. So one of the salient facts about the HK 36 is that it was, indeed, the last of a long line that began with the Mauser Werke StG 45. For that, as well as its innovative ammunition and concept, it deserves to be remembered.

We are aware that this post is far from comprehensive, but we think it tells the story of this rare experiment to the extent that it’s been made public. If there is a single thorough article on the HK 36 in the Intertubes somewhere, we did not find it. The best and most authoritative sources, based on factory information, are those 1975 Jane’s and National Defense articles, and three short pages in Full Circle, which reproduces much of the ND article’s content.