One option is to revert to 7.62×51 completely, but to field a low-recoil loading like that used in the Japanese Howa Type 64 automatic rifle, which fires a 138 grain bullet at 2,300 fps (a slight improvement on the AKM round). This could become the standard rifle round with the option of switching to full-power ammo at long range: the full-power ammo would of course be retained in MGs and DMRs anyway. This solution has its attractions, but the disadvantages would be the need to adjust the rifle’s gas system when switching between ammo types, the need to check each magazine when reloading to ensure that it contains the appropriate ammo, and a doubling in rifle ammo weight without doing anything to reduce the weight of MG ammunition.
At the moment, neither the U.S. nor British armies seem interested in changing from the current 5.56mm and 7.62mm cartridges, but are these really the best we can do? The evidence suggests they are not.
The Characteristics of a General-Purpose Cartridge
The most exhaustive recent attempt to improve on the 5.56mm was the development of the 6.8×43 Remington Special Purpose Cartridge, or SPC. A primary requirement was to keep its overall length within that of the 5.56×45, so that it could be used in adapted AR-15 guns. To provide the extra propellant capacity required to deliver a worthwhile increase in performance the case had to be wider, and the old .30 Remington case was taken as the basis. Much experimentation with calibres ranging from 5.56mm to 7.62mm followed, and the 6.5mm seemed to be the best compromise until the 6.8mm was tried and proved to be the optimum choice for this case.
If what we want is the best available replacement for the 5.56mm at short to medium ranges, then the 6.8mm Rem. fits the bill better than anything else. The author has test-fired the 5.56mm HK 416, HK 416/6.8mm and 7.62mm HK 417 in quick succession, and the 6.8mm generates only a small increase in recoil over the 5.56mm; the gap between the 6.8mm and the 7.62mm is much more noticeable, even with the higher weight of the HK 417. Basically, the 6.8mm is controllable enough for short-range burst fire to be useful, the 7.62mm is not.
However, the need to restrict the 6.8mm’s overall cartridge length has resulted in a short and relatively light bullet. The 6.8mm still offers a better long-range performance than the 5.56mm M855, but falls well short of the 7.62mm M80 so could not replace the larger calibre. A slightly different choice of case, calibre and bullet could make a huge difference to the long-range performance while still keeping the ammo weight, energy and recoil in the same ballpark as the 6.8mm. This is clearly demonstrated by the next attempt to produce a better-performing round to fit the AR-15 family, the 6.5×38 Grendel from Alexander Arms. This uses a shorter but fatter case to leave enough room for a long, low-drag bullet which loses velocity more slowly than the 7.62mm M80. Both the 6.8mm and 6.5mm are midway between the 5.56mm and 7.62mm in ammo weight and muzzle energy.
This graph (above) of bullet energy in foot pounds at different ranges illustrates these points. It compares the 7.62mm M80 (147 grain at 2,700 fps) with the 5.56mm M855 (62 grains at 3,100 fps), the 6.8mm Rem (115 grains at 2,700 fps) and the 6.5mm Grendel (123 grains at 2,600 fps) when fired from 20 inch barrels (results in 14.5 inch barrels follow the same pattern). Two things are obvious: first, that the initial performance gaps between the 5.56mm and the 6.8mm, and between the 6.8mm and the 7.62mm, become steadily wider with increasing range. The second point is that the 6.5mm performs differently; while starting with a muzzle energy similar to the 6.8mm, its more aerodynamic bullet allows it to match the 7.62mm at long range. The 6.5mm bullet is also less affected by wind drift than the others; a 10 mph cross-wind at 1,000 m deflecting the 7.62mm by 161 inches, the 5.56mm by 230 inches, the 6.8mm by 197 inches and the 6.5mm by 118 inches. These 6.5mm results were obtained with the use of a specialised target bullet, but a ball round of the same weight and shape shouldn’t be far behind.
This indicates the way to obtain a long-range performance to match the 7.62mm M80 with significantly less ammunition weight and recoil: by using a smaller-calibre but low-drag bullet fired at a medium muzzle velocity. The 6.5mm Grendel shows one way to do it, but similar principles could be applied to 6.8mm calibre (perhaps with a 130 grain bullet at around 2,500 fps) or 7mm (140 grains at 2,400 fps). The bigger calibres would hit harder, but weigh more and generate more recoil.
Other important points concerning the bullet design of any new cartridge are that it must yaw rapidly and reliably on impact in order to maximise its terminal effectiveness, and also penetrate intermediate barriers without being deflected. Armour penetration is of no significance in the current type of conflict, but separate AP ammunition would have to be developed and available in case of need.
The above design principles apply to both traditional and advanced ammunition types. If an entirely new gun and ammunition system based on the current caseless/plastic-cased LSAT project were to be adopted, it would be a terrible waste of an opportunity if the merits of an intermediate calibre were not evaluated. It would also be desirable to develop a conventional intermediate cartridge first, both to test the effectiveness of the calibre and to act as a fall-back in case LSAT fails.
What might a new conventional cartridge look like? The designs of both the 6.8mm Remington and 6.5mm Grendel were constrained by the need to be short enough to fit into 5.56mm actions; a new cartridge, for a new family of weapons, should not face this limitation. One final observation: some people argue that the fighting in Afghanistan is not typical, so if we dropped the 5.56mm in favour of a more effective intermediate cartridge we would be equipping for the last war, not the next one. However, current thinking in both the British and U.S. Armies is that asymmetric warfare against insurgent forces will remain the most probable type of conflict in the future. That means “boots on the ground”, with infantry warfare and weapons remaining important. The less stable parts of the world, where such conflicts are likely to take place, often have areas with little vegetation and few buildings where there is lots of scope for long-range fire. Besides, what would be the disadvantages of changing calibre even if future combat is at shorter ranges? We would still benefit from ammunition that is much more effective than 5.56mm at any range, and lighter than 7.62mm: advantages well worth having in any circumstances. This is especially so considering that full-power 7.62mm rifles and MGs are still in common use in many parts of the world, which puts troops armed with 5.56mm weapons at an increasing disadvantage as the range lengthens.
None of this is rocket science. We could have gone down this intermediate calibre path decades ago, first with the U.S. .276 Pedersen in the 1930s then with the British 7×43 in the 1950s, and we should not, yet again, ignore the opportunity to do so with our next generation of small arms.
About the Author
Anthony G Williams is an independent consultant, co-editor of Jane’s Ammunition Handbook, and co-author with Maxim Popenker of “Assault Rifle: the Development of the Modern Military Rifle and its Ammunition”. He maintains a website at www.quarry.nildram.co.uk.