​How Multipurpose Saved the .50 Cal

It’s not very often that you encounter something that works extremely well, but at the same time even its developers can’t tell you exactly why. But that is the case with multipurpose ammunition. For the first 25 years of its existence, its developers did not fully understand every aspect of the physics behind it. They only knew that it worked, and that it worked very well, providing broader target effects than previously thought to be possible.

In a simple comparison, conventional detonating ammunition contains a high explosive that is detonated by a second very quick explosive that creates the sharp detonating impulse. By contrast, multipurpose ammunition uses a pyrotechnic fuse, which provides what’s known as ‘deflagration,’ where the explosive grains burn more as particles to detonate the high explosive.

The simplified process is likened by some to comparing the effects of a popcorn popper versus a heating oven. Instead of one microsecond that a detonator might require to ‘pop off’ the high explosive energy, a pyrotechnic fuse might use one millisecond to do the same thing. It’s still fast, but that millisecond allows the round to proceed another one-half meter within the target prior to detonation.

Responding to the Threats of the Cold War

Testing multipurpose at Raufoss in the 1960s.

Much of the early effort was instigated by Norway’s receipt of F-104 and F-5 fighters in the 1960s and the resulting need for ammunition optimized for that country’s unique geographic defense missions.

At the time, one of the anticipated threats involved the hostile use of heavy fishing trawler-type boats, possibly up-armored, that could deliver initial strike elements along Norway’s shoreline.

Previous options for engaging these types of targets from the air had included the use of either conventional high explosive ammunition, which could explode outside of an armor-plated target, or armor piercing ammunition, which made a hole in the target but had little effect after that.

According to Gard Ødegårdstuen, one of the lead engineers on multipurpose ammunition development at Nammo AS, Norwegian defense planners were looking for something that would do more than just put a hole in the boat but also have fire or explosion effects within the boat.

‘And by luck they discovered that this new pyrotechnically-initiated ammunition had the desired delayed action,’ he said. ‘And to be honest, that was an observation that they did not expect.’

One reason for the surprise was the long-held belief that you needed some type of detonator to set off a delayed explosion. But through experimentation it was discovered that it was possible to set off explosives by slower pyrotechnic burning. Moreover, the slower process also had additional target effects of fragmentation.

Watch as one of the original designers of the Multipurpose describes its invention.

Enter the Computers

Investigation of the multipurpose ammunition concept started at Raufoss with early experimental work in 1967. Engineers believed at the time that they understood the process, but in hindsight some acknowledge that they were challenged to describe some elements of the physics behind it.

But the concept clearly did work, and the experimentation eventually led to a 1992 research program effort split between the Norwegian Defense Research Establishment (FFI) and then Raufoss (today part of Nammo) to try to increase that understanding of those physics. It is significant to note that the intervening years were marked by the proliferation of computer technology and that the computers now allowed the modeling of things like deflagration, which served to increase overall understanding of the multipurpose design.

Engineer at work at Raufoss during the 1990s.

However, while Ødegårdstuen also began working on the project in the early 1990s, he is quick to credit the earlier work of Kåre Strandli and the Raufoss engineering team in the 1960s and 1970s and their calculations, measurements and documentation of with far simpler equipment than what is available today.

‘To be fair, after several years Kåre and the team understood much of it,’ he said. ‘But without computers they were not able to do all the calculations. So there was some theory where they were not quite sure, but the computers also verified much of what they already understood. They may have filed in a little bit more in some corners but in principle I think they understood quite well.’

The bottom line is that, even before computers, the Raufoss engineers were able to create an ammunition technology that remains viable and effective more than a half century later.

That effectiveness is reflected in the fact that more than two dozen countries now use some form of the technology in their defense inventories, with more poised to join their ranks.

An unexpected visit

One representative example can be found in the United States, where a widely-shared story involves a US Navy officer who suddenly appeared at the Raufoss headquarters reception desk in 1976.

He was assigned to the US Navy Test Center at China Lake, California, and had been in Sweden for meetings. He decided to stop by Raufoss to ask how to best demilitarize 20mm multipurpose ammunition – 1000 rounds to be exact, that had been transferred to his facility.

After some internal investigation, it was discovered that one enterprising director at Raufoss some years earlier had provided the 1000 multipurpose rounds to the US in the hopes of having them tested, and then hopefully procured. That desired testing never happened, with the ammunition instead being transferred from one facility to another, and the current holders at China Lake having no idea what to do with it, or how to get rid of it. They did not know its composition, how it worked, or anything else about it.

The Raufoss staff convinced the US Navy representative to have the ammunition tested instead of destroying it. And the Navy did perform that testing, with such great results that, in the end, the US first obtained a license to produce multipurpose ammunition for US fighters, before outright buying the rights to produce their own version of 20mm and 25mm multipurpose ammunition. To this day it is the main explosive ammunition used by US F-15, F-16, F-18, and AV-8 jets.

Along with the 20mm and 25mm, the multipurpose concept has also been applied to development of 27mm, for possible application on the Gripen or Typhoon fighters, 30mm, for both aircraft and fighting vehicles, and development of a 40mm design.

Saving the .50 cal

Manufacturing .50 cal ammunition at Raufoss.

But perhaps the biggest success can be seen in the .50 caliber, where the pyrotechnic detonation design is credited by some with breathing new life into an aging weapon system where the bullet size was considered to be ‘too small’ for a traditional detonator.

‘Before multipurpose there was only ball ammunition for .50 cal,’ Ødegårdstuen observed. ‘But now suddenly, in the early 1980s you had something of a high explosive round option in that caliber. That was very new. And the users loved it.’

The early technology investigations of multipurpose ammunition designs are also feeding current activities, including the 25mm Armor Piercing Ammunition Explosive (APEX) round developed for the F-35. The program started in 2003 and, although using a mechanical fuse with delay, builds on lessons learned and the fundamental understanding of physics and energetics derived from investigations over the previous decade.

Looking further into the future, Ødegårdstuen predicts that multipurpose ammunition will continue to play a key role because of benefits it provides in effectiveness, simplicity and lower expense when compared to many alternative designs.

‘Of course there will be advancements in high level capabilities,’ he said. ‘And we are certainly looking to stretch the multipurpose concept a little bit more. So my colleagues and I believe that multipurpose will always be there.’