DSEI UK 2025: SIG details 5.56 Hybrid

Defence procurement is often set in the context of threat or adversary capabilities. The Soviet Union’s numerical superiority, for instance, drove NATO’s focus on precision strike as a means to improve the efficacy of each munition. In the modern day, advances in body armour are driving a new look at assault rifle ammunition. For the US Army, this led to the 6.8 mm round and SIG’s hybrid cartridge enabling it to be fired at the velocities needed to defeat a given level of body armour at range. Now, SIG has applied that same technology to the widely used 5.56×45 mm NATO standard cartridge, it’s called the 5.56 Hybrid. Calibre Defence met with Duncan Stewart, director of business and programme development for SIG UK, and Robby Johnson, Chief Marketing Officer, during DSEI 2025 to learn more about the new cartridge, and what it could bring to users. 

“Ammunition is the thing that reaches out and touches the adversary, and there’s only so far you can go with the rifle to improve performance,” Robby explained, as we took a break from the crowds to talk at the SIG stand. “So, the next step is increasing range, penetration, and so on to get better performance.” Robby’s point is an interesting one; of the six rifles reviewed by Calibre Defence that could enter Project GRAYBURN, the UK’s effort to replace the SA80, four were based on the AR-pattern assault rifle. All are available with differing barrel lengths, picatinny rails, and have more or less the same magazine capacity. 

“For the past 12 months we’ve been playing with what the 5.56 hybrid would look like. We knew that it would be a path forward, but that customers might not be sure, and we are now seeing that demand signal,” Robby explained, referring to the 6.8×51 mm hybrid cartridge selected by the US Army. 

In a nutshell, the hybrid cartridge adds a steel base to a standard brass cartridge, which allows the cartridge to be loaded to much higher pressures because the steel strengthens the cartridge. “You could put any projectile into it and it will give 20-30% more energy,” Duncan told me. Robby continued, “for programmes like GRAYBURN, you can gain about 200 m/second, combine that speed with the right projectile and it becomes more possible to penetrate body armour.”

Specifically, the hybrid 5.56 mm round has an operating pressure of around 80,000 psi while a standard 5.56×45 mm round is in the region of 62,000 psi. “You can put any projectile in the 5.56 hybrid cartridge and deliver it at 970 m/s from an 11 inch barrel, exceeding speeds you would normally get from an SA80 with a 20 inch barrel,” Duncan said. This is an important data point because, generally speaking, the longer the barrel, the longer the gasses from the cartridge have to impart energy to the projectile. So, a longer barrel will generally increase range and speed of a round. Gaining the same from a shorter barrel opens up a lot of options for an armed force. 

This extra speed enables rounds fired from the 5.56 hybrid cartridge to penetrate 0.6 cm of steel at 225 metres, compared to 75m for standard US 5.56 rounds. At 225 metres an average height human is quite a small target; imagine holding an AA battery out at arm’s length – that is roughly equivalent to the size of a human target at 225 metres, assuming they are stood up. Modern assault rifles carrying advanced optics can achieve enough accuracy at this range for infantry to be effective, but beyond that is stretching the capabilities of most riflemen without specialist training. So, in theory, the 5.56 hybrid cartridge provides enhanced lethality at the upper limits of useful assault rifle range. 

Project GRAYBURN and the hybrid 5.56

Having recently secured an order for its MCX Spear assault rifle with the Royal Marines, SIG is positioning its MCX family for Project GRAYBURN. That procurement, which is expected to result in orders for 177,000 rifles over at least 10 years, has drawn attention from several competitors. The Labour Government has placed a lot of emphasis on defence as a driver for economic growth. However, despite being one of the largest potential assault rifle orders in Europe, the current lack of manufacturing capability in the UK means that the economics of the programme may not favour domestic production of rifles.

“There’s always an aspiration to have in-country production, but there are some realities around the viability of that, and SIG is exploring all options. But, we know the requirement is 177k rifles, and it is likely they will have to phase the inflow and funding profile,” Duncan explained. The current in-service SA80 is approaching an unavoidable end to its service life, as keeping the rifles in service will rapidly become uneconomical. So, the British Army is likely to phase the acquisition of replacements in line with the withdrawal of the old rifles. 

“You can’t easily start and stop rifle production lines to meet a demand in a way that is economically prudent, but ammunition is a constant requirement and requires steady production,” Robby explained in reference to SIG’s approach to Project GRAYBURN. “ If you focus on the ammunition production, then parts, and suppressors etc. you can be self-sustaining.  You will know and control your demand – which parts need replacing and manufacturing – and scale to meet your own demand economically ,” he added. 

Duncan continued, suggesting that, “it makes a lot more sense for the UK to localise production of ammunition than the guns…we are well-placed to do that, and have an early partnership with NAMMO.” However, the GRAYBURN requirements are still unclear, so Duncan continued to state that, “SIG can provide a scaled localisation offer, depending on the requirements and fleet numbers. In the grand scheme of things, however, there are probably more important strategic capabilities that the UK needs.” 

In terms of the requirements for the rifle, “the Army has said that they want to be able to defeat a specific body armour at a given range. SIG has pretty much the only TRL9 ammunition product  that can defeat that armour at the moment, using 7.62/7.62 hybrid without a specialist projectile, and if you add in 5.56 hybrid, then it builds the offering yet further,” Duncan said. 

Calibre comment

Small arms production is one of the many things that the UK’s MoD is no longer able to source domestically. After the SA80 production was finished, the factories producing the rifles closed. Re-establishing that type of production capability is expensive, especially when it comes to barrel manufacturing. So, if the UK government insists on domestic production, it will likely add significantly to the cost of Project GRAYBURN. The more rifles ordered, the more economically viable domestic production becomes, and the greater the returns to the UK economy. However, this is one programme in which the MoD might have to make greater moves to meet industry where it is, rather than demanding compliance with its various industrial strategies. 

By Sam Cranny-Evans, published on September 16, 2025. Credit for the lead image is PO (Phot) Sean Clee/Crown Copyright. It shows a handful of standard 5.56×45 mm ammunition.