Are ballistic helmets bulletproof?
Notes on the protective properties of combat helmets.
20th century steel helmets have very poor resistance to small arms threats; the PASGT, ACH, and most “IIIA”-rated helmets will stop virtually all pistol caliber threats, but will not stop rifle rounds; the ECH and IHPS will stop some rifle threats, but will not reliably stop all or even a majority of them, and helmet shell deformation may cause the shell to come into contact with the wearer’s head, potentially causing serious injury; future helmets will reliably stop steel-cored rifle ball threats with minimal helmet shell deformation.
– The ballistic helmet was never designed to offer protection from small arms fire.
– The steel helmets of WWI and WWII — which were in use by the US Military until the early 1980s, and by European forces well into the 1990s — were developed to protect against indirect fire, such as mortar and shell fragments. These helmets were not officially rated to stop any handgun or rifle projectile.
– The first aramid helmets such as the PASGT were more effective at stopping fragments and shrapnel, but were likewise not rated to stop any small arms threat. It was, however, discovered that these aramid helmets would stop service handgun ammunition, like the 9x19mm FMJ.
– Correspondingly, the ACH helmet — an improved aramid helmet introduced in 2003 and designed to build upon the successes of the PASGT whilst discarding the design flaws that were uncovered over more than a decade of use — was specified to stop 9x19mm FMJ service ammunition at up to 1450 fps. The US Military’s Inspector general noted in a report to Congress that “[t]he ACH is not designed to provide ballistic protection from threats more lethal (for example, higher velocity, or larger mass) than a 9mm FMJ RN.”
– The ECH helmet program, begun in 2009, was instituted to see whether a helmet can be developed that would stop the prevalent rifle threats in Iraq and Afghanistan. Results have been mixed, as will be described further herein, but the project has met with success for the most part.
– Near-future advances in helmet engineering will enable helmets to stop even steel-cored rifle threats.
An understanding of the combat helmet’s ballistic properties can only follow from this: That the combat helmet was developed as a military tool. Military planners knew well that indirect fire from mortars and artillery can inflict terrible casualties, and the first helmets were designed and issued to counter those particular threats.
In WWI, explosive or fragmenting munitions were responsible for roughly 60-70% of all combat casualties.  At the battle of Verdun, fragmentation and shrapnel from artillery bombardment caused at least 70% of the approximately 800,000 casualties that both sides suffered. The remainder were, for the most part, inflicted by relatively heavy rifle and machine-gun rounds which even the best helmets of today would not be able to stop. 
The first helmet of the war to enter mass production and see widespread use — and the first modern combat helmet — was the French casque Adrian. This was made of mild steel, 0.7 to 0.8mm thick, with a tensile strength of at least 415 MPa and moderate ductility. (18% tensile elongation.) This helmet was capable of resisting a 230-grain, .45 caliber ball round at 400-450 feet per second, which is roughly half the .45 ACP’s muzzle velocity. But notwithstanding this poor performance against bullets, it is estimated to have defeated 75% of all shrapnel impacts from airburst munitions, and it had, therefore, an immediate positive impact on troop casualty rates and morale. In the Adrian’s wake, every other participant in WWI — except for Russia — hastened to develop and issue steel helmets of their own. Like the Adrian, these helmets had very poor resistance to small arms impacts, but were highly effective at protecting their wearers from shrapnel and fragmentation.
These same steel helmets, with minor modifications in some instances, were employed by all American and European forces through WWII. And here they proved even more vital, for whereas fragments and shrapnel accounted for approximately 65% of all WWI casualties, they accounted for 73% of WWII’s wartime wounds. The widespread use of the steel helmet shifted patterns of wounding and was highly effective at preventing fatal head injury. When the war was over, it was calculated that of all hits upon the US military’s M1 helmet 54% were defeated and, in fact, of all incapacitating hits upon the body, the M1 helmet prevented 10% of them. 
Needless to say, all of the helmets of the war were totally incapable of stopping 8mm Mauser, 7.62x54mmR, or .30-06 bullets at most engagement distances — and in fact they would, invariably, fail to stop 7.62x25mm Tokarev handgun/submachinegun rounds within 100 yards under normal ballistic test conditions — but that wasn’t their intended function.
So it is interesting that these steel helmets were often tested against down-loaded handgun rounds for quality control purposes.  For although helmets were designed to defeat fragments and shrapnel, solely, the use of fragment simulating projectiles was not yet practical. Simulating fragments and shrapnel is a difficult problem which requires specialized test projectiles, and the appropriate methodologies had not yet been developed at that time.
The M1 was typically tested against .45 caliber ball ammunition. Its ballistic limit against 230gr. .45 caliber ball ammunition with a gilding metal jacket and soft lead core was 761 to 911 feet per second, and it was noted that the front of the M1 helmet should reliably stop such ammunition at 35 yards.
.45 caliber helmet testing cartridges
Against 230 grain .45 caliber ball ammunition with a copper-clad steel jacket and “hard” lead core, the M1 helmet’s performance was considerably reduced, so that it’s ballistic limit was between 542 and 735 feet per second, depending on location of impact and other factors.
Against 124gr. 9mm FMJ ammunition with a muzzle velocity of 1250 feet per second, the M1 was reliably penetrated out to 130 yards and possibly beyond. The test range that the Military used at the time didn’t extend any further than that. 
Interestingly, the soft, large, and extremely heavy .45 ball ammo that was used as the test projectile for the M1 couldn’t possibly have been more different from the fragment-simulating projectiles (FSP) used to test helmets today. The FSPs are much lighter — ranging from 2 to 64 grains — and they’re made entirely of AISI 4340 steel heat-treated to 30 HRC. With no jacket, no deformable lead core, and much lighter weights and lower diameters, they’re a qualitatively different threat in every respect.
In any event, despite objectively poor performance against handgun threats, the M1 steel helmet was standard-issue for US soldiers for decades. Remaining in service until the early 80s, it is by a wide margin the longest-serving US military helmet. But serious efforts to replace it with an improved helmet had begun in the 1960s, and ran through the 1970s.
At first, different bulk materials like titanium alloys and polycarbonate were considered, and hundreds of prototypes were made. The titanium helmets were deemed too expensive — and their performance, though better than the extremely cheap M1, wasn’t considered a significant enough improvement. The polycarbonate helmets, though relatively impressive in terms of performance, were removed from contention on account of their poor resistance to solvent and chemical exposure. Ultimately, these projects and many others did not result in any helmet material that might replace the M1’s WWI-vintage Hadfield steel. (Incidentally, many of these efforts — including the polycarbonate helmet — were performed under the LINCLOE Project, which ultimately resulted in the ALICE gear system.)
Fiberglass helmets were also prototyped and examined, and although they exhibited superior ballistic properties in comparison with the M1 helmet, they lacked durability and were prone to delaminating in salt water. They were not adopted by the US military, but the first ballistic police helmets were of fiberglass and were modeled upon these early US military experiments.
In the late 1960s, Brigadier General George Hayes, working at the Office of the Surgeon General, spearheaded an effort to replace the M1 with a nylon-laminate helmet of a closer-fitting and somewhat more streamlined design. Prototypes of this new fiber-composite helmet were produced in the Army’s Edgewood Arsenal by a Mr. George Stewart, and the so-called Hayes-Stewart helmet was slowly evaluated over the next several years. Its ballistic properties were not improved over the M1, its area of coverage was only a slight improvement, and it was “not properly human factors engineered” particularly in that it made wearing a flak jacket uncomfortable. For these reasons, the helmet was not adopted by the Army and work ceased by 1972.
Hayes-Stewart nylon laminate helmet
But the Hayes-Stewart helmet spurred general interest in polymeric fiber composite helmets, and in this respect it can be seen as a fore-runner to the PASGT helmet, which was developed later in the 1970s.
In the mid 1960s, duPont chemists working on materials for automobile tire reinforcement identified a high-modulus polymer fiber which was first named PRD-49-IV was later trademarked and sold as Kevlar® 29. This material was of immediate interest to the US military. For at the time of its production it was 2.5 times as strong as any other textile fiber, and its performance was 60-100% better than ballistic nylon on a weight basis. Little time was wasted in replacing the nylon and fiberglass flak jackets with more protective and lighter Kevlar vests. And, taking a page from the Hayes-Stewart, Kevlar-laminate helmets — stiffened with about 20% by weight of a polymeric (PVB-phenolic) resin — were developed. Both the vests and the helmets were introduced as the PASGT program, and were issued to the troops in 1983. Some U.S. soldiers wore PASGT helmets in Grenada (Operation Urgent Fury) in 1983, Panama (Operation Just Cause) in 1989, and in the Middle East (Desert Shield/Desert Storm) in 1990-1991.
Unlike the M1, the PASGT helmet was tested against real FSPs, and, with a V50 against 17gr. .22 caliber FSPs at 2000 feet per second, exhibited considerably better performance than the M1, which had a V50 of around 1000 fps.  Against the entire range of FSPs, from 2 grains to 64 grains, the PASGT’s performance was anywhere from 42-79% superior to the M1.  The PASGT, though not officially rated to stop handgun rounds, was also demonstrably capable of stopping 9mm FMJ service ammunition at typical muzzle velocities.
All of this is tempered somewhat by the fact that the PASGT helmet is markedly heavier than the M1. A size XL PASGT weighs 4.2 pounds; a size XL M1 weighs 2.85 pounds. (The M1 was only offered in one size, which corresponds to an XL in dimensions and coverage.) Were the M1 made 47% heavier, thicker, out of a more modern steel alloy, it stands to reason that its protective capabilities could have kept pace, at a much lower cost and with superior performance against small-arms projectiles. Indeed, we know that this is the case, for a modernized steel helmet — the Adept NovaSteel — is simultaneously lighter than the PASGT and performs better against both fragments and handgun rounds. It is frankly surprising that something along such lines was never attempted or, seemingly, considered. As things stand, it could be argued, and very convincingly, that the introduction of the Kevlar helmet was a mistake.
And that’s without taking into consideration the fact that the PASGT was perhaps an order of magnitude more expensive than the M1, which cost the military $3.03/unit in the early 1950s. ($1.05 for the manganese steel shell, $1.98 for the liner.)
Cost of the infantryman’s kit, US Army, early 1950s
In any event, the PASGT helmet was adopted, and was received quite favorably overall. There was some grumbling about its shape, with a brim which caused significant reductions of field of view when compared with brimless helmets, and there were numerous complaints about its webbing harness system, which was both uncomfortable and exhibited extremely poor blunt impact performance.
The Modular Integrated Communication Helmet (MICH) was a PASGT-derivative project spearheaded by USSOCOM that sought to correct those deficiencies. It kept the aramid shell, but removed the brim, made slight changes to the overall geometry of the shell to better enable the use of communications gear, and replaced the webbing with foam padding. The MICH was also slightly lighter than the PASGT — in part on account of changes to its geometry, and in part due to slight advances in aramid technology and composite processing methods that allowed for a lower volume fraction of resin.  The MICH was received extremely favorably, and, with some minor modifications, was adopted by Army as the Advanced Combat Helmet (ACH) shortly after its introduction. The ACH became the Army’s primary combat helmet in the mid 2000s.
The MICH and ACH, unlike the PASGT, were rated to stop handgun threats. The ACH specification demands, as a condition of lot acceptance, that helmets stop the 124gr. 9mm FMJ at 1400+50 fps. Backface deformation limits were set at 16mm for the sides and crown, and 25.4mm for the front and rear of the helmet. The ACH’s performance against fragments is improved by 10% over the PASGT, with a minimum 17gr. FSP V50 at 2200 feet per second. 
The ACH, MICH, and PASGT are all — like the steel helmets of old — generally incapable of reliably stopping rifle rounds. The Army’s Inspector General, in a 2013 report to Congress on the performance and capabilities of the ACH, noted: “The ACH is not designed to provide ballistic protection from threats more lethal (for example, higher velocity, or larger mass) than a 9mm FMJ RN. Field data indicate that the ACH performs well against its intended threats, but is penetrable from rifle threats that are most commonly seen in theater. A new product called the Enhanced Combat Helmet (ECH) is currently under design and development to defeat threats more lethal than a 9mm FMJ RN.” 
The ECH program began in 2009, with a mandate to produce a helmet with a 35 percent increase in fragmentation protection and protection from certain rifle threats common in Iraq and Afghanistan — at the same weight as the ACH.  This was deemed possible with the utilization of UHMWPE fiber composite materials, which were at that time enabling very light armor plates, and had been in use in very lightweight French military helmets — such as the CGF Gallet ”SPECTRA” helmet — for nearly two decades.
By late 2010 and through 2013, many ECH helmet prototypes had been produced and submitted to the Army. Overall, performance against fragments was 53% better than the ACH, performance against the 9mm FMJ was roughly 10% better, and performance against a certain rifle round was 153% better.  It must, however, be noted that these numbers are not perfectly unambiguous. Against the 9mm FMJ threat, the ECH had to comply with helmet backface deformation requirements — that is, it had to stop the round with less than 16mm/24.5mm backface signature onto a clay headform. Against the rifle threat, those backface deformation requirements were deemed “too restrictive.” So there was no requirement at all, and testing was performed on a pass/fail basis where, even should the helmet utterly cave in, it would still “pass” if the projectile were stopped.
The ECH, at $840/unit, was also exactly three times more expensive than the ACH, which cost the US military $280/unit.
In light of these facts, the Operational Test & Evaluation Office of the Secretary of Defense (DOT&E) recommended that the Army not buy or field the ECH. They held that the unit cost is too high and that Soldiers wearing the ECH would have an unacceptably high risk of death or severe injury from excessive backface deformation from rifle threat bullets. The Army Office of the Surgeon General — which, decades before, had spearheaded the Hayes-Stewart and PASGT helmets — concurred with DOT&E’s assessment and recommendations.
In a subsequent 2014 report on the state of the ECH program, the US Navy noted that “while the ECH protects against perforation by the specified small arms threat, it does not provide a significant overall improvement in operational capability over currently-fielded helmets against the specified small arms threat. The deformation induced by the impact of a non-perforating small arms threat impact exceeds accepted deformation standards across most of the threat’s effective range. The ECH is therefore unlikely to provide meaningful protection over a significant portion of the threat’s effective range. The ECH provides improved fragmentation protection compared to the fielded Advanced Combat Helmet and the Light Weight Helmet (LWH).
“[..] It is unknown, definitively, whether the ECH provides protection against injury when the deforming helmet impacts the head. There is, however, reason to be concerned because the deformation induced by the impact of a non-perforating small arms threat exceeds accepted deformation standards (established for a 9 mm round) across most of the threat’s effective range.” 
The ECH was nevertheless fielded in limited numbers, and has been quite favorably received by troops and command. Insofar as the single most common small-arms threat in theater was the 7.62x39mm MSC ball round, and insofar as the ECH is capable of stopping that round, the introduction of the UHMWPE helmet was a success. That the ECH has extremely good resistance to high-velocity fragments must also be noted as a strong point in its favor.
The new IHPS appears to be to the ECH as the ACH was to the MICH. It is a helmet system currently in development that appears to be intended for general issue — as a total replacement for the ACH — that incorporates most of the features of the ECH. As of this writing (Nov 2021) it is being fielded in limited numbers. Like the ECH, it is made of UHMWPE, and its ballistic capabilities are seemingly identical to those of the ECH. The IHPS specification, like the ECH specification, expressly notes that backface deformation is to be measured when the helmet is tested against 9mm FMJ projectiles, but not measured when tested against rifle rounds. 
IHPS helmet with multicam cover
The exact ballistic capabilities of the ECH and IHPS have not been disclosed to the public, but it is believed that both helmets are capable of stopping 7.62x39mm and lead-cored 5.56x45mm ball rounds at muzzle velocity, and also offer standoff protection from 7.62x51mm M80 ball. Both the ECH and IHPS are readily available on the military surplus market, and some units have been subjected to impromptu testing.
Neither the IHPS nor the ECH will stop rifle rounds with a hard or semi-hard steel core. This includes the common 5.56x45mm M855, the 5.56x45mm M855A1, the 7.62x54mmR LPS, the 7.62x39mm API-BZ, the 5.45x39mm 7N6, and many other projectiles which are presently in use with military and paramilitary forces worldwide. In fact, the standard-issue rifle rounds of all developed countries are steel-core rounds. Fiber composite materials, when used as standalone armor materials, generally offer limited protection against such threats, whether they are used in body armor, helmets, or static barriers. 
The primary differences between the IHPS and the ECH are aesthetic. The ECH was built to look like the ACH, whereas the IHPS is a novel design with a totally proprietary accessory-mounting system. (And, though outside the scope of this particular article, which focuses on ballistic characteristics, the IHPS also has a slightly higher blunt impact rating than any other military helmet ever fielded. Its more efficient padding system gives it a small performance advantage over the ECH and ACH.)
ECH-style helmets are currently being produced for the civilian and police market. The Ops-Core FAST RF1  and the Highcom/XTEK Striker Arditi  are examples of this. The performance characteristics, material construction, and weights of these new helmets are broadly in-line with those of the ECH and IHPS. They are likely tested in the same way, i.e. with no backface deformation limit. Under the circumstances, this is not inappropriate.
Just as the MICH and ACH cured the deficiencies of the PASGT, near-future future helmets are going to solve the two major gaps in the performance of the ECH/IHPS: The backface deformation problem, and the steel-cored bullet problem. How they are going to do this is trivially self-evident: The solution can only lie in the use of hard, but very lightweight, ceramic materials affixed to the helmet’s outer surface. This ceramic layer will disrupt or shatter incoming steel-cored projectiles, to such an extent that the underlying helmet shell will be able to catch and absorb whatever remains of them and their residual kinetic energy. Helmets have already been produced that have stopped the 5.56x45mm M855A1, with its hardened steel penetrator, at over 3000 feet per second with backface deformation measured at under 10mm. On the inner surface of the helmet, exceedingly stiff materials limit shell deformation to within the levels set down in the ACH specification, even when those helmets are struck by high-energy rifle fire.
Such solutions are already commercially available, e.g. in ceramic up-armor tile kits that are already commercially available.  This sort of system design reliably stops steel-cored rifle ball rounds, and, at the same time, helmet backface deformation upon impact is not excessive — it is, in fact, within the limits set for the ACH against 9mm FMJ handgun rounds.
So here we have the evolution of the ballistic capabilities of the combat helmet: The tale begins with steel helmets with moderate fragment resistance and very little resistance to small arms projectiles. Later, aramid helmets with improved ballistic and anti-fragment capabilities were introduced. (But these helmets were significantly heavier and much more expensive — and a similarly weighty steel helmet likely would have offered similarly improved ballistic performance. These facts should make the introduction of the aramid helmet, in the first instance, somewhat controversial.) These aramid helmets are capable of reliably stopping 9mm FMJ at 1450 fps, as well as all lesser handgun threats. Very recently, UHMWPE has supplanted aramid as the helmet material of choice, and helmets made of this material offer much better resistance to penetration from fragments and small arms threats — including rifle threats with lead or mild steel cores. These helmets do, however, deform excessively when struck by rifle rounds, to such an extent that the helmet shell can come into contact with the skull at high speed, so the actual degree of small arms protection they offer is, as yet, unknown. Future helmets shall extend small arms protection to steel cored rifle ball rounds, and will mitigate, to some extent, the severe backface deformation seen in most of today’s “rifle resistant” helmets.
Combat helmets, traditionally, are not exactly bulletproof — but they’re getting closer all the time, and helmets capable of stopping all common small-arms threats are right on the horizon.