Frustratingly for the intermediate-calibre supporters, the US Army realised after initial experience in Vietnam that they had made a mistake and cancelled further production of the 7.62mm M14 rifle (which had anyway experienced serious production quality problems). Inspired by experimental work which showed the efficiency of small-calibre rifles, they went to the other extreme in adopting the M16 rifle and its tiny .223 (5.56x45) cartridge (after some competition from the .224 Winchester and two different .25" Winchesters (6.35x48 and 6.35x53). The 5.56 was developed from Remington commercial hunting rounds which had been designed for taking small game such as rabbits. This was actually only intended to be an interim purchase pending the perfecting of the SPIW flechette rifle (see below) but as this never happened, the 5.56x45 became the US Army's standard rifle cartridge by default. 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 and penetration - although its terminal effectiveness on human targets has been even more critically questioned. The USA has continued to develop improved 5.56mm ammunition, recently adopting the lead-free M855A1 Enhanced Performance Round and (for the USMC) the MK318 Mod 0.The author also addresses the issue of the "ideal" cartridge:
Rather surprisingly, the Russians were inspired by the 5.56mm to develop a new 5.45x39 7N6 cartridge for their next-generation rifle, the AK74. This is a bit less powerful than the 5.56 NATO although the bullet does have an exceptionally good aerodynamic form achieved partly by a hollow tip, giving it a good performance at long range. There is a small lead element at the back of the tip which moves forward on impact, causing the bullet to destabilise quickly. Despite this feature, it is understood that in some quarters the older 7.62mm M1943 round is still preferred.
More recently, the Chinese have introduced a 5.8x42 calibre for assault rifles and LMGs. The ballistics seem little different from the 5.56mm, although it is claimed that it outperforms it, with penetration superior to the SS109, a flatter trajectory, and a higher retained velocity and energy downrange. The differences are only marginal, however, as the standard rifle round is only loaded to 41,500 psi chamber pressure, compared with 55,000-62,000 for the 5.56x45. Furthermore, the emphasis in the bullet design has been the penetration of body armour; its hardened steel core will punch through 10mm armour plate at 300m, which is in the same class as steel-cored 5.56mm AP rounds. A heavier loading of the 5.8x42 also exists, for use in the GMPG and sniper rifles. The most recent development is the introduction of a new, universal loading of this cartridge (designated DPB-10), intended to replace the earlier versions. This has a 4.6g bullet at 915 m/s for (71 grains at 3,000 fps).
One conclusion as a result of recent combat experience is that the 5.56mm weapons are most effective in short-range combat. That was satisfactory in Iraq which mainly saw urban fighting, but was revealed as a major deficiency in the much longer ranges common in Afghanistan. The British and US Armies both found that the 300-400m maximum effective range of 5.56mm weapons was inadequate when foot patrols were engaged by small groups of Taleban, using 7.62x54R SVD rifles and PKM LMGs at ranges of up to 900m. As a result, 7.62mm rifles and MGs have made a comeback at section level in the foot patrols.
In addition, performance of the small-calibre, high-velocity rounds (especially the 5.56mm NATO) is erratic; sometimes they work well, sometimes they don't, depending on their impact velocity, the precise manufacturing details and the angle at which they strike the target. There is more on the subject of small-arms terminal effectiveness HERE. The 7.62mm weapons are more reliably effective but are much heavier (both guns and ammunition), and the recoil of the rifles is also heavy, making automatic fire uncontrollable.
A larger-calibre, more powerful cartridge than the 5.56mm, but still significantly lighter than 7.62mm and generating light enough recoil to permit controllable automatic rifle fire when required, might also deliver another substantial benefit: its performance could be close enough to that of the 7.62mm NATO to permit the new cartridge to replace both existing 5.56mm and 7.62mm rounds, providing considerable benefits in reducing the weight of MG ammunition plus the costs of small-arms acquisition, training and support. The August 2011 report by the US Army's PEO Soldier report titled Soldier Battlefield Effectiveness includes a number of points in favour of general-purpose weapons and ammunition, summarised concisely in this:"Ultimately, Army service rifles must be general purpose in nature and embody a series of tradeoffs that balance optimum performance for a wide range of possible missions in a range of operating environments. With global missions taking Soldiers from islands to mountains and jungles to deserts, the Army can’t buy 1.1 million new service rifles every time it’s called upon to operate in a different environment."Is it possible to achieve a suitable general-purpose cartridge? The evidence suggests strongly that it is. Most recently, a 2010 investigation into rifle calibres by the US Army's ARDEC determined that, when considered across a range of criteria, the 6.5mm to 7mm calibres provide a better solution than 5.56mm or 7.62mm.
The British aimed for this with the 7x43 cartridge half a century ago, and by all accounts succeeded admirably. This gives us an upper calibre limit. It seems unlikely that a cartridge with the long-range performance to replace the 7.62mm can be achieved with anything smaller than 6.5mm calibre, which gives us the lower limit. We need to specify a bullet sectional density ratio of at least .230 and preferably .250 in order to retain velocity better than the 7.62mm (whose 9.33g bullet has an SDR of 0.217 - the 5.56mm SS109 bullet has an SDR of 0.174,and the new 5.0g Mk 262 is 0.220) 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. This works out as the following range of choices with minimum bullet weights in common calibres:
7mm/.276": bullet weight 8.4-9 g (130-140 grains) at 772-745 m/s (2,532-2,440 fps) = 2,500J6.86mm/.270": bullet weight 7.9-8.5 g (124-131 grains) at 788-760 m/s (2,585-2,500 fps) = 2,455J6.5mm/.258": bullet weight 7.25-8 g (112-122 grains) at 820-780 m/s (2,690-2,560 fps) = 2,430J
Any of the above options would do, but for the sake of argument let's take the 6.5mm. A cartridge of this performance could be smaller in length and diameter than the old military 6.5mm rifle rounds such as the Arisaka, the Carcano and the Mannlicher, with a diameter of around 10.6mm and a length of about 45mm. In fact, the case diameter and length would be similar for all of the cartridges in the list above. As a matter of interest, the four standard case diameters manufactured in the West and adopted by most cartridges in this class are approximately 9.6mm (5.56 NATO), 10.6mm (6.8mm Remington), 11.3mm (6.5mm Grendel), and 12.0mm (7.62mm NATO).
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So, we have our ideal military general-purpose assault rifle and MG cartridge - and we could have had it many decades ago. What are the chances of such a cartridge being adopted now? Some hopes were raised recently by the introduction of a couple of new rounds which (more or less) fit the above criteria. One 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); not too different from the 'ideal' 6.86mm listed above. The cartridge case is based on the old .30 Remington commercial round, with a diameter of 10.6mm, intermediate between the 5.56x45 (9.5mm) and the 7.62x51 (11.9mm). Overall length is kept within the 57mm limit to fit in the M16 action, which limits the length of the bullets which can be loaded, blunting their long-range performance. Even so, 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 (the SD is 0.214). The trajectory matches that of the 7.62x51 M80 ball out to 500m, and is only 10cm low at 600m. The development of this round was sponsored from within the US SOCOM (Special Operations Command) who were looking for a more powerful cartridge than the 5.56mm, and it has reportedly been successfully tested in action, although not adopted for logistic and financial reasons. SOCOM, and now the USMC, has instead adopted the 5.56mm MK318 Mod 0 SOST ammunition, which is reportedly much more effective but uses an open-point bullet unacceptable to UK and probably other European countries. In any case, while the 6.8mm Remington would be a significant improvement over the 5.56mm, it does not have the range to replace 7.62mm and thus could not be a general-purpose cartridge.
More recently, another challenger emerged in the form of the 6.5mm Grendel (6.5x38).This uses a slightly fatter case (the same 11.3mm diameter as the 7.62x39 Russian) which enables it to be shorter, thereby leaving space for longer and more aerodynamic bullets. This enables it to fire a 123 grain Lapua Scenar bullet at 2,530 fps (8.0g at 770 m/s: 2,370J) from a 16 inch barrel, with a far superior ballistic coefficient to the 5.56mm Mk 262 or 6.8mm bullets (SD 0.252). In a longer rifle or MG barrel this provides trajectory and velocity loss figures to match or better those of the 7.62x51 M80 ball round. This clearly has potential for a general-purpose cartridge, but there are some reservations. The performance shown below is achieved with a high-quality target bullet; a military ball round with a crimping cannelure would not have such a good ballistic coefficient. Also, adopting a lead-free design would significantly lengthen the bullet (and a tracer even more so) so it would have to protrude more deeply into the case, reducing the propellant capacity. And the shape of the case may be rather stubby to be ideal in a belt-fed MG.
The designs of both the 6.8mm Remington and the 6.5mm Grendel have been compromised by the need to keep within the 57mm overall length of the 5.56x45, so that existing 5.56mm weapons can be rebarrelled to chamber them. A clean-sheet design would probably result in an overall length of around 65mm.Read the whole thing.
Another new round currently being developed by Cris Murray, who was involved in the 6.8mm Remington development, is the 7x46 UIAC (Universal Intermediate Assault Cartridge). This is based on the same 11.3mm case diameter as the 6.5mm Grendel. In its proposed standard loading it fires a 130 grain bullet at 2,650 fps (8.4g at 810 m/s). This develops 2,750 J, so it is very much at the top end of the size, calibre and power range. The extra case width over the 6.8mm Rem's 10.6mm might however prove useful in accommodating long, lead-free bullets and possibly thicker cases made from polymer, should these succeed in meeting military requirements. Interestingly, the basic case dimensions of the UIAC are very similar to those of the .270 British from the late 1940s, and the performance is very similar to that of the .276 Pedersen of the late 1920s.
Of the current rounds, the 7mm UIAC and the 6.5mm Grendel represent the top and bottom of the range for a general-purpose rifle/MG round in terms of their performance, recoil, calibre, size and weight. Perhaps the ideal lies somewhere in between?