Brennan and Alexander
Assault Rifles and Their Ammunition: History and Prospects
January 11, 2004
Anthony G. Williams, author of Cannon, Machine Guns, and Ammunition: A Website for Military Gun and Ammunition Enthusiasts, (www.quarry.nildram.co.uk). Reprinted by permission.
First, I need to define what I mean by an assault rifle, as there are various definitions around. The one I use is: A military rifle, capable of controlled, fully-automatic fire from the shoulder, with an effective range of at least 300 metres.
This has some clear implications for the ammunition such weapons are chambered for. First, it excludes all weapons designed around pistol cartridges (i.e. sub-machine gunsSMGs) as they only generate around 500 joules muzzle energy and cannot meet the range requirement. Second, it excludes the traditional full power military rifle/MG cartridges such as the .303, the .30-06, the 7.92x57 and the 7.62x51 NATO (typically firing 10-12g bullets at 750-850 m/s, and developing around 3,000-4,000 joules), as these are so powerful that their recoil is uncontrollable in fully-automatic fire from the shoulder. Assault rifles therefore need to be designed around a cartridge intermediate in power between pistol and full-power rifle rounds; in practice, in the 1,250-2,500j range depending on the calibre.
Attempts have been made to extend the effective range of SMGs by developing more powerful cartridges for them. However, there is a limit to the degree to which this can be achieved as the basic API blowback mechanism used by most SMGs is not suited to high-powered ammunition. Attempts have also been made to design automatic rifles around full-power cartridges, perhaps the most famous example being the German FG 42 paratroop rifle in 7.92x57. Some of the early rifles in 7.62x51 NATO, such as the American M14, were also capable of fully automatic fire, but the recoil problem made them incapable of accurate fire on full-auto and they cannot be classified as assault rifles.
There are basically two approaches to designing a suitable intermediate cartridge with the appropriate compromise between long range and light recoil. One is to retain the same 7.6-7.9mm calibre as the full-power round, but with a shorter cartridge case firing a lighter bullet at a lower muzzle velocity (lets call these full calibre assault rifle, or FCAR, rounds). The other is to reduce the calibre while retaining the same, or a higher, velocity (reduced calibre, or RCAR rounds). Please click here for a photo gallery of all the rounds discussed in this article.
FCAR rounds score well in the traditional methods of measuring stopping power (which are dominated by calibre and bullet weight) and also by being less affected by the bullets striking foliage etc on their way to the target. However, they have a relatively steep trajectory and a rapid velocity loss due to the short, fat bullets, which quickly reduces their effectiveness at long range.
A decision to reduce the calibre raises the immediate question: By how much? At the large end of the RCAR scale (7mm), bullet weight and muzzle velocity can be much the same as in the FCAR cartridges, but the better ballistic coefficient due to the longer and more slender bullet will reduce velocity loss and improve long-range performance. However, there is potentially some loss in stopping power. As the calibre decreases, so the recoil and the ammunition weight become lighter and the velocity can be higher, thereby flattening the trajectory; all good things. The downside is that the stopping power becomes more controversial (relying on velocity rather than calibre and bullet mass; which according to combat reports sometimes works, sometimes doesnt) and the long-range performance begins to decrease again as small-calibre bullets generally have poorer sectional density ratios, and thereby ballistic coefficients, than large-calibre ones.
Different nations have made different choices in developing assault rifles, and the purpose of this article is to describe and analyse them in order to examine the future prospects for this type of weapon.
Development Before World War 2
The elements of an assault rifle were in place surprisingly early in the history of automatic weapons. Self-loading rifles were developed before the end of the 19th Century and the first selective fire (semi or full auto) rifle using a medium-power cartridge was probably the Italian 6.5mm Cei-Rigotti, developed between 1900 and 1905, but this was not adopted.
The first service weapon which can be identified as conforming to the specification of an assault rifle dates back to the First World War; the Russian Federov Avtomat of 1916. This was a selective fire weapon using a short-recoil action and was chambered for a military rifle cartridge of intermediate calibre and powerthe 6.5x50SR Arisakalarge quantities of Japanese rifles in this calibre having been acquired by Russia. This was an excellent choice, as the cartridge combined moderate recoil with a good long-range performance, but only about 3,000 Avtomats were made. They were used in action in the Russian Civil War and thereby earned their place in small-arms history.
Elsewhere at this time, the prevalence of trench warfare and the associated close fighting had focused attention on short-range automatic weapons, in complete contrast to the prewar obsession with accurate long-range rifle fire. This resulted in three different lines of development: pistols which were modified with longer barrels and stocks and sometimes adapted to fully-automatic fire; purpose-designed SMGs; and the Pedersen Device (which replaced the bolt in the US Springfield Rifle with an automatic mechanism to fire small .30 cal (7.62x20) rounds developing less than 400 joules; it was never used in anger). Interest in assault rifles disappeared from view until the Second World War.
Pistol-based carbines were a natural extension of the occasionally recurring fad for equipping pistols with detachable shoulder stocks in order to permit more accurate aiming. Longer barrels further extended the effective range (partly through increased velocity, partly because of the longer sight base) and so weapons such as the Mauser C96 and P08 produced carbine derivatives, usually only capable of semi-automatic fire. These were relatively expensive to make, however, so the future in short-range automatics lay with the much simpler API blowback SMG. The first of these (if you discount the curious twin-barrel Villar Perosa) was the Bergman MP18 in 9x19 Parabellum calibre, which was the ancestor of the Thompson, the MP 38/40, the Sten Gun, the PPSh and so on.
Attempts to improve the power and range of the small automatics, such as the use of the 9x25 Mauser Export round in the Solothurn and Kiraly SMGs (which saw some service), and the development of the Mannlicher Carbine in 7.62x32 and of the Swiss 7.65x35, did not catch on. In fact, despite the evidence that most shooting during WW1 was at short range, armies continued to show an interest in full-power rifle/MG rounds. The Japanese Army even planned to replace their 6.5x50SR cartridge with a new 7.7x58 calibre, although they never completed the changeover. The Italians were similarly caught at the start of WW2 part-way through a change from their 6.5x52 Mannlicher-Carcano rifle to a 7.35x51 calibre.
Why was this? Probably because the need for a full-auto rifle was generally resisted, on the grounds of economy (automatic rifles being much more expensive and requiring more maintenance than bolt-action ones), and also the fear that soldiers would just spray ammunition around at a great rate, causing increased cost and supply problems (this latter concern was, of course, fully justified, but has been addressed by improving supply arrangements). So even the one nation wealthy enough to afford an automatic riflethe USArestricted the M1 Garand to semi-auto fire, and full-power rounds biased towards MG use prevailed. Incidentally, the USA did of course have the Browning Automatic Rifle in service, but that was too heavy to be a rifle replacement and was used as a light machine gun.
There had been some efforts towards considering intermediate calibres, with the US Ordnance Board sponsoring comparative trials in the early 1930s of the effectiveness of different rifle cartridges using anaesthetised pigs and goats to assess wounding effectiveness. They concentrated on a .25 (6.35mm), a .276 and the existing .30. The .25 (8g at 820 m/s, for 2,700 joules) most impressed the testers, but the Board chose the .276 Pedersen (7x51) a medium-power round developing 2,400 joules, which would have made an effective assault rifle cartridge. At this point, the top brass insisted on the new rifle being chambered for the .30-06 (7.62x63), so another opportunity was lost.
There was one rather odd American development not followed by any other countrythe M1 Carbine. This was a light, semi-automatic rifle chambered for an intermediate, straight-cased 7.62x33 round. It was not originally intended for front-line troops, but more as a self-defence weapon for second-line units, on the sensible grounds that it was much easier to shoot accurately than a pistol. The M2 version came with a full-auto option, and thereby comes close to our definition of an assault rifle, but the cartridge was rather weak and the light, blunt-nosed bullet lost its modest velocity too quickly.
World War 2 and After: The Assault Rifle Emerges
The modern line of assault rifle development started with the Germans, with the MP43/44 (later renamed StG44 for Sturmgewehr or assault rifle). This followed the FCAR route, shortening the usual 7.92x57 K98 rifle/MG case to 33mm, and loading a lighter bullet at a reduced velocity. Keeping the same calibre was a matter of production convenience; it had previously been calculated that a 7mm calibre would be ideal. Despite initial opposition from Hitler, this was the weapon the Army wanted to back-up their MG42 GPMGs, and it was produced and used in quantity. However, the end of the war stopped the direct line of development of this significant weapon.
Next to emerge was the Soviet Simonov SKS, made in large numbers in various countries but now almost forgotten due to the fame of its successor, the Kalashnikov AK47. Both weapons were chambered for a new 7.62x39 M1943 cartridge, but the SKS was not an assault rifle, being capable of semi-automatic fire only. There is still some sensitivity about the connection between the AK47 and the StG44, but two things are clear; despite the apparent similarity, the AK47 was not a direct copy as it uses a quite different mechanism, but on the other hand Kalashnikov and his team must have known about the StG 44 and it is difficult to believe that they were not influenced by it.
The AK47 and its ammunition (also used in the RPD light MG) so dominated the assault rifle field until the late 1960s that it is sometimes difficult to remember that there were other developments, one of which saw service. This was the Czech vz52 rifle chambered for their 7.62x45, a superior cartridge to the AK47s in terms of range, but it was soon replaced by the vz52/57 (chambered for the 7.62x39) in the interests of commonality with the rest of the Warsaw Pact. The vz52 was only semi-auto, but the Czechs were working on a selective fire weapon based on the round when the changeover to the Russian calibre took place; this assault rifle was the vz58. Other nations also experimented with short-case FCAR rounds, particularly the French and the Swiss. Cartridges such as the Swiss 7.65x38 MP and 7.5x38, the French 7.65mm Model 48 (7.92x35the French also experimented with calibres up to 9mm), and the unusual Spanish 7.92x40 CETME Model 53 (which used a lightweight but highly streamlined bullet) were all unsuccessful contenders during the 1950s.
Another which very nearly saw service was the British EM2 bullpup rifle, initially chambered for a new 7x43 cartridge (later slightly modified as the .280/30) which fired its 8.4g bullet at 730 m/s, for a muzzle energy of 2,240 joules. Unlike the AK47, which continued to be supplemented by the full-power 7.62x54R Nagant cartridge in MGs and sniper rifles, this was a carefully-judged attempt to produce a weapon which could replace both the 9mm SMG and the full-power .303 rifle in one compact package. A GPMG based on the Bren mechanism but with belt feed, the TADEN, was also developed to use this round and replace both the Bren and the Vickers MMG. It appears to have been very successful and other NATO countries (Canada and Belgium, at least) were very interested in the concept. The British and Belgians made great efforts to meet the objections of the US Army, who thought it wasn't powerful enough, first by stepping up the loading to 2,700 joules, then by developing a longer cartridge (the 7x49which actually saw service with Venezuela in the FN FAL rifle). Despite this, the Americans insisted on NATO adopting a common round which had to be of .30 calibre and powerful enough to replace the .30-06 in MGswhich meant by definition that it could not be used in an assault rifle. So the 7.62x51 was adopted, which apart from being half an inch shorter than the .30-06 cartridge represented no progress whatsoever over this fifty-year old design. American experiments were made in the late 1950s with a range of smaller calibres, such as the .22/30 NATO, the .25 Winchester (6.35x48), the .25/30 NATO (6.35x51) and the .27 NATO (6.85x51), but these led to nothing.
Frustratingly for the intermediate-calibre supporters, the US Army realised after initial experience in Vietnam that they had made a mistake and went to the other extreme in adopting the M16 rifle and its tiny .223 (5.56x45) cartridge, developed from Remington commercial hunting rounds which had been designed for taking small game such as rabbits. Much controversy arose about its effectiveness in stopping a determined enemy, but what was clear was that the long-range performance of the little bullet (designated M193) was poor. In the next competition for a new NATO rifle cartridge held in the late 1970s, the 5.56mm was duly adopted but in the new Belgian SS109 loading (M855 being the US version), which has a heavier bullet at a lower muzzle velocity and thereby achieves a better long-range performance.
Rather surprisingly, the Russians followed suit and adopted a new 5.45x39 7N6 cartridge for their next-generation rifle, the AK74. This is no more powerful than the 5.56 NATO although it does have the uplifting feature of a hollow bullet tip, designed to bend on impact and thereby encourage tumbling in the target's body. Despite this attempt to maximise the wound severity, it is understood that in some quarters the older AK47 round is still preferred.
More recently, the Chinese have begun to introduce a 5.8x42 calibre for assault rifles and LMGs. The ballistics seem little different from the 5.56mm and 5.45mm weapons, although it is claimed that it outperforms both of them, with penetration superior to the SS109, a flatter trajectory, and a higher retained velocity and energy downrange.
Finally, the 5.7mm FN has achieved some sales, in both the FiveSeven pistol and the P90 SMG. However, despite its improved range performance, this 5.56x28 cartridge only develops 550 joules and is really a 9mm pistol round replacement, so doesnt qualify as an assault rifle.
Despite the domination of the 5.56mm NATO round (in much of the world) and the Kalashnikov family (in the rest), experiments with new assault rifle and ammunition concepts have of course continued, even with the occasional competition being held. Some of the experiments have been with conventional ammunition, others have been more exotic.
Perhaps the most interesting and instructive series of experiments took place in the UK in the late 1960s, when thorough attempt was made to design an ideal military small-arms round. This started with calculations of the bullet energy required to inflict a disabling wound on soldiers with various levels of protection. The energy varied depending on the calibre, as a larger calibre required more energy to push it through armour. For example, it was calculated that while a 7.62mm bullet would need 700 joules to penetrate modern helmets and heavy body armour, a 7mm would require 650j, a 6.25mm 580j, a 5.5mm 500j and a 4.5mm 320j (this last figure looks wrong and should probably be 420j). This figures applied at the target; muzzle energies would clearly have to much higher, depending on the required range and the ballistic characteristics of the bullet.
A range of optimum solutions for ballistics at different calibres was produced. These resulted in muzzle energies ranging from 825 joules in 4.5mm to 2,470j in 7mm. More work led to a preferred solution; a 6.25mm calibre with a bullet of 6.48g at 817 m/s, for a muzzle energy of 2,160 joules. The old 7mm EM2 case was necked down to 6.25mm for live firing experiments, although had the calibre been adopted a new cartridge would probably have been designed. Tests revealed that the 6.25 cartridge matched the 7.62 NATO in penetration out to 600m and remained effective for a considerably longer distance, while producing recoil closer to the 5.56mm.
As related in The .256 British, at much the same time, the US Army was looking to develop a new squad automatic weapon (SAW). The 7.62mm was too powerful, the 5.56mm didn't have a sufficiently long range, so a 6x45 round was developed which proved satisfactory but was not adopted because of concerns about putting a multiplicity of calibres into service. The Russians in recent years have also offered weapons in a new 6mm calibre which appears to be considerably more powerful than the 5.45mm, but no sales seem to have been secured so far.
The Swiss experimented with at least two cartridges in the late 1970s before adopting the 5.56mm NATO; the 5.56x48 Eiger and 6.45x48 GP 80. The 5.56mm fired a 3.7g bullet at 1,050 m/s for 2,040j (considerably more than the 5.56mm NATO) while the 6.45mm managing to propel its 6.3g bullet at 900 m/s for 2,550j. With the benefit of hindsight, a heavier bullet at a more moderate velocity might have provided a better general-purpose loading for the 6.45mm.
Despite concerns about the stopping power of the 5.56mm, some experimenters have worked with even smaller calibres. The British proffered a 4.85x49 (actually, 5mm) round for the NATO contest which chose the 5.56mm, the H&K G11 (described below) used a 4.7mm. Calibres of 4.6, 4.3, 3.5 and 3mm (and possibly, more or less) have been tried, mainly during the 1960s and 1970s. It is difficult to imagine that such cartridges could do anything to improve on the 5.56mm's range and stopping power. There is also the capillary problem with the really small bores; any water which gets into the barrel will be difficult to dislodge.
The more exotic experiments have proceeded in different directions, with different aims in mind. Some attempts have been made to improve the hit probability of conventional 7.62mm cartridges with multi-ball loadings, using two (duplex) or three (triplex) lightweight bullets stacked on top of each other. One of these, the US M198 duplex, was even accepted for service. A salvo-squeezebore (firing several stacked conical projectiles which were squeezed down to a smaller calibre by a muzzle attachment) was developed for the .50 BMG, but versions which have appeared in 7.62mm NATO appeared to have been made for their novelty value.
Others have attempted to achieve the same aim by using flechette technology (in principle, a scaled-down APFSDS tank gun round: APersFSDS?) to achieve an extremely short flight time and flat trajectory resulting from a muzzle velocity of around 1,400 m/s. This gives such weapons an almost ray-gun like performance, with allowances for range, wind-drift and target movement being hardly needed at normal battle ranges. This was first seriously proposed in the American Special Purpose Infantry Weapon project of the late 1960s, in which several manufacturers produced weapons using basically similar ammunition firing 1.8mm diameter darts. Accuracy was not as good as conventional rifles, however, and the cost of the ammunition was very high. Attempting to achieve everything in one weapon by building in a grenade launcher didn't help, either, and the project foundered. One hopes that the same fate doesn't befall the OICW.
Flechette weapons were revived by two of the competitors in the Advanced Combat Rifle (ACR) contest of the late 1980s. This contest was intended to improve the poor hit probability achieved by average soldiers in the stress of battle, which using the M16 was only guaranteed (pH = 1.0) at up to 45m, and dropped to a pH of 0.1 (one shot in ten) by 220m. The theory was that firing three slightly dispersed shots in quick succession should enable the pH to be doubled, and several different weapon concepts were prepared.
The Colt ACR contender was simply an improved M16A2 firing a duplex cartridge, H&K submitted the caseless G11, while AAI and Steyr offered weapons firing flechette rounds, the Steyr ammunition being plastic-cased. Although all of the weapons apparently performed well and did increase the hit probability, none of them managed to double it.
Finally, there was at least one attempt at a multi-flechette weapon; the .330 Amron Aerojet, which contained three flechettes within its 8.38x69 light-alloy case.
Other experiments have looked at different cartridge types to suit novel gun designs. Perhaps the most bizarre was the folded path ammunition, stemming from a desire to make the cartridge as short as possible to speed up the firing cycle. These were made in many calibres, including 5.56mm. Another try was the Hughes Lockless (also made in calibres up to 30mm) which concealed the bullet within a flat, rectangular plastic case. This was designed to slot sideways into a simple gun action.
The closest to adoption of all of the exotics was the caseless cartridge, in the form of the Heckler & Koch G11 rifle. It was actually about to be adopted by the German Army to replace the 7.62mm G3 (Germany never having adopted the 5.56mm NATO) when the Cold War ended and the Berlin Wall came down. Military re-equipment spending promptly halted. H&K were financially ruined by the cancellation of the G11 and fell into the hands of Royal Ordnance, where they have earned their keep by sorting out the long-running problems of the British Army's SA-80 rifle, but thats another story\\\\\\\\u2026
Caseless ammunition has obvious benefits. It is much lighter and more compact (no metal case), and it is unnecessary to arrange for the extraction and ejection of the fired case (perhaps the principal source of weapon jams). The disadvantages are that it is much more vulnerable to damage (which H&K got around by supplying the ammo in sealed plastic see-through packs which clipped directly to the gun) and the propellant is more likely to cook-off in a hot chamber; a problem exacerbated by the fact that a brass cartridge case makes an efficient job of transporting heat from the gun. Despite this, H&K (or rather Dynamit Nobel) cracked the problem by developing a new heat-resistant kind of propellant and produced a battle-worthy weapon.
Despite all of these experiments, new weapons currently planned for adoption are relatively conventional, at least as far as the assault rifle element is concerned. The US Army currently appears to favour the M4, a carbine version of the M16 with a shorter barrel. The H&K G36 is probably the current market leader and has been adopted in modified form as the rifle element of the OICW. The G36 is also forming the basis of the XM8, which is being developed to replace the M16/M4 family. Both of these weapons are going against the trend by having a conventional instead of bullpup layout. This is preferred by many as it is easier to switch sides and use left-handed, but it carries the penalty of a shorter barrel for the same overall length. Compactness for manoeuvring weapons in AIFVs or helicopters, or for use in close-quarters fighting, is evidently considered important as most new developments are bullpups. These include the Israeli Tavor, the SAR-21 from Singapore and the FN2000. The last of these (a modular system, as is common nowadays, with various add-ons being optional) gets over the left-handed problem by ejecting the spent cases forwards and downwards.
All of these weapons are chambered for the usual 5.56x45 NATO. The problem with short barrels in this calibre is that they reduce the muzzle velocity, and the 5.56mm bullets rely on a high impact velocity to tumble and fragment. At lower velocities the bullets will not fragment and much of the wounding potential is lost. In the normal 20" (51 cm) barrel the fragmentation distance is around 200m, but in the short carbine barrel it can be as low as 50100m, depending on the bullet. The controversy over the effectiveness of the M4 in Afghanistan matches that of the reliability of the luckless British SA80.
The USA has been developing the OICW (Objective Infantry Combat Weapon), also known as the SABR and the XM29. This combines a short-barrelled 5.56mm with a self-loading low-velocity 20mm gun. The heart of the weapon is a laser rangefinder coupled to a fire-control computer linked to optronic sights and an electronic fuze-setter. This complex and extremely expensive fire control system means that the gunner can fire a 20mm shell to explode directly over the target at anything up to 1,000m. However, it has not proved possible to reduce the weight from 8.2 kg to the target 6.8 kg, so development has been temporarily shelved in favour of the XM8 and the XM25, which is a 25mm self-loading grenade launcher. The French are experimenting with a similar (and even bulkier) system, the PAPOP, which has a 35mm grenade element, while the Australians are basing theirs on the Metal Storm technology, in which the grenade shells are stacked within the barrel. Such systems are undoubtedly impressive but whether they will still work when they are several years old, especially after having being kicked around a combat zone for a few weeks, remains to be seen.
What conclusions can we draw from all this, and what does the future hold?
Two conclusions about the present situation seem pretty clear. One is that there would be financial and logistical benefits in having only one military rifle/MG cartridge. The other is that it wouldn't hurt to have a rifle cartridge with more reliable hitting power than the 5.56mm. As it happens, both conclusions point in the same direction; towards a cartridge intermediate in power between the 5.56mm and 7.62mm NATO.
Such an ideal cartridge would need to combine a long-range effectiveness comparable with the 7.62mm, with a recoil light enough to permit controlled, full-auto fire. Is it possible to achieve this? The evidence suggests strongly that it is. The British aimed to do this with the 7x43 cartridge half a century ago, and by all accounts succeeded admirably. This gives us an upper calibre limit. I don't think that a useful increase in performance over the 5.56mm can be achieved with anything smaller than 6mm calibre, which gives us the lower limit. We need to specify a bullet sectional density ratio of about .230 in order to retain velocity better than the 7.62mm (whose 9.33g bullet has an SDR of 0.217the 5.56mm SS109 bullet has an SDR of 0.174) and thereby deliver the long-range performance we want. We also need a muzzle energy of no more than 2,500 joules to provide the right balance of power and recoil. Taking into account that smaller calibres need less energy to penetrate armour, this works out as the following range of choices in common calibres:
7mm/.284": bullet weight 8.4g (130 grains) at 770 m/s (2,525 fps) = 2,500j
6.85mm/.270": bullet weight 7.9g (122 grains) at 784 m/s (2,570 fps) = 2,430j
6.5mm/.264": bullet weight 6.9g (106 grains) at 820 m/s (2,690 fps) = 2,330j
6.35mm/.25": bullet weight 6.5g (100 grains) at 834 m/s (2,736 fps) = 2,260j
6mm/.243": bullet weight 5.9g (91 grains) at 854 m/s (2,800 fps) = 2,150j
Any of the above options would do, but for the sake of argument let;s take the 6.5mm. A cartridge of this calibre would be smaller than the more powerful 6.5mm Arisaka. If a similar case diameter were retained then length could be reduced to about 45mm. In fact, the case diameter and length would be similar for all of the above cartridges.
So, we have our ideal military general-purpose assault rifle and MG cartridgethe 6.5x45 GPand we could have had it many decades ago. What are the chances of such a cartridge being adopted now? Until very recently, it would have been reasonable to assess these as zero; too many resources have been invested in the current weapon systems to throw all of those out of the window and start again. However, recent reports indicate that the US SOCOM is testing a more powerful cartridge designed to fit in the M16 action. This is the 6.8x43 Remington SPC (Special Purpose Cartridge) which fires a 115-grain bullet at 2,650 fps from a 16.5 inch barrel (7.45g at 808 m/s = 2,430j); very similar to the ideal 6.85mm listed above. The cartridge case is based on the old .30 Remington commercial round, with a larger diameter than the 5.56x45 to increase the case capacity. This round develops 55% more muzzle energy than the 62-grain SS109/M855 loading at the muzzle, rising to 84% better at 550m due to its superior ballistic coefficient. Even standard 5.56mm magazines can be used, with some modifications to the lips and follower nd with capacity reduced from 30 to 25 rounds. If this is adopted for US Special Forces and proves successful, then it stands a chance of being used more widely. This is the most promising development in military rifle ammunition for about half a century, and clearly has the potential to replace both the 5.56x45 and 7.62x51 rounds. Time will tell if this is yet another false dawn.
In the long run, it seems likely that current cartridge technology will be replaced by something quite different, although it is not immediately apparent what this might be. Flechette rounds have been tried and rejected, at least for now, despite their impressive long-range performance (the Steyr ACR round started at 1,500 m/s at the muzzle and was still travelling at over 1,200 m/s at 1,000m). Caseless ammunition has been tried and rejected, at least for now. We may see something more radical; perhaps a large-calibre weapon folllowing on from the current grenade projects and capable of firing a cluster of flechettes, an HE shell, or a variety of other lethal or non-lethal natures.
Looking even further ahead, perhaps someone will crack the energy supply problem and deliver an electromagnetic weapon capable of extremely high velocities at an acceptable size and weight. Time will tell, but it could be very long time before we see the last of the conventional 5.56mm weapons.