An experimental high-velocity ammunition could breathe new life into the legacy M4 carbines used by special operations forces.
A U.S. Special Operations Command solicitation posted recently on the government’s contracting website calls for vendor proposals on a “Hypervelocity Improved Capability Assault Rifle,” or HICAR – a carbine system with an improved or swapped out upper receiver capable of firing developmental high-velocity rounds to an effective range of at least 600 meters; this is double the range than is provided by the M4’s current upper receiver.
Hypervelocity is defined as speeds of 3,000 meters or more per second, which is very roughly triple the speed of legacy 5.56 NATO ammo, which the M4 currently uses.
The solicitation sheds light on the military’s continued interest in hypervelocity rounds, a concept that has been the subject of test and speculation for years, but has yet to make it out of the lab.
As the news site TechLink explained in 2019, hypervelocity weapons open up two possibilities: first, the lethality at extended range from standard-sized personal weapons; and second, conventional rifle velocities from much shorter and more compact guns – for example, from a 10-inch rifle barrel.
The sticking point is the pressure required to fire ammunition at hypervelocity. As TechLink noted, this requires significant weapon redesign and comes with tradeoffs.
“To withstand the increase in chamber pressure and still be able to cycle, the standard bolt and locking lugs mechanism that holds the cartridge would need to be substantially increased in size and weight – adding to the already overburdened soldier,” author Troy Carter wrote.
The new solicitation aims to “integrate advancements in material science and weapon design” to crack the code on building an upper receiver that can withstand the pressure and bring hypervelocity ammo into the realm of reality.
According to specifications, the desired new rifle will be capable of firing ammo loaded to at least 82,000 pounds per square inch, compared to about 52,000 PSI chamber pressure for a legacy carbine. It will have a barrel of 11-12 inches, compared to 14.5 inches for the M4.
And SOCOM doesn’t want to add a ton of weight; the rifle can’t exceed eight pounds unloaded, compared to the 6.4-pound weight of an unloaded M4.
The white papers from manufacturers need to include an estimated cost per weapon, an expected timeline to deliver prototypes and a description of any risks foreseen with weapon development.
In a May interview with The War Zone, SOCOM’s program manager for Lethality, Army Lt. Col. Alan Wood, explained the appeal of HICAR and why a juiced-up M4 with double the effective range might be a better weapon than the rifles that have largely replaced the M4 in recent years, including the Army’s 6.8mm M7 and Sig Saeur’s lightweight XM8 carbine.
“The Green Berets who do a lot of partnering with foreign SOF operators, but there’s no [6.8mm] ammo running around central Africa, or you know, pick your favorite spot where we like to do partnering operations around the world,” Wood told The War Zone. “But there’s a lot of 5.56mm and other common cartridges of that nature, and so what we want to… achieve is maybe not the same effect as what the Army’s done with the M7 and the M8 and that high-velocity ammunition that they have, but where could we get close to that in 5.56 and potentially other calibers in the future.”
He added that among the key tech developments bringing a hypervelocity M4 closer to reality were new ammunition alloys that perform better than legacy brass and are available more cheaply.
Related: SOCOM’s potential new firearm is a revolution
In late May, the Army inked a licensing agreement with Federal Ammunition to use the company’s steel-cased Peak Alloy technology for “multiple cartridges and weapons systems.” The agreement comes with an understanding that Federal Ammunition will deliver 40 million cases of the ammo. Described as “a high-strength, proprietary steel alloy casing that enables significantly higher chamber pressure – in excess of 80,000 PSI” the ammo was first introduced in 2025 with the 7mm Backcountry rifle.
And more materials are in development. In 2025, Lehigh University published findings about a collaboration with the Army Research Lab to develop a super-strong copper-based alloy that can hold shape under extreme pressure.
“By merging the high-temperature resilience of nickel-based superalloys with copper – which is known for exceptional conductivity – the material paves the way for next-generation applications, including heat exchangers, advanced propulsion systems and thermal management solutions for cutting-edge missile and hypersonic technologies,” researchers wrote.
SOCOM expects to host a HICAR “pitch day” in September in which attendees will be provided with experimental M855A1+ hypervelocity rounds as well as the standard-issue 5.56 M855A1 Enhanced Performance Round. They’ll fire about 600 demonstration rounds, then immediately transition to a 30-minute pitch presentation to explain their concepts.
Feature Image: STUTTGART, GERMANY (April 16, 2021) A Special Forces soldier assigned to 10th Special Forces Group reloads his M4 carbine during a live-fire range at Panzer Kaserne. Marksmanship training ensures that soldiers are able to fire pistols and rifles accurately under a variety of conditions. (U.S. Navy photo by 1st Lieutenant Robert Kunzig)
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