"Concerns within the US military about the 5.56 mm cartridge have spread beyond individual complaints. A group within SOCOM has promoted the development of a new cartridge that could replace the 5.56 mm: the 6.8 x 43 mm Remington Special Purpose Cartridge (SPC). This fires a larger and heavier (7.45 g) bullet, which has proved to be considerably more effective in tests even when fired from carbine-length barrels. It has been specifically designed to maximise lethality within normal rifle ranges of up to 300 m, although in fact the trajectory and terminal effects rival those of the 7.62 x 51 mm M80 ball out to 500 m.
A rival approach has been the fielding in limited numbers of the new 5.56 mm Mk 262 cartridge, with a heavier (5 g) bullet designed originally for long-range target shooting. This not only has a better long-range performance than the SS109/M855, it also fragments out to longer ranges. However, at 600 m the remaining energy of the 6.8 mm bullet is over 40 per cent greater than that of the Mk 262. Despite this power, the recoil of the 6.8 mm is much less compared to the 7.62 x 51 mm and the cartridge is compact enough to fit into adapted 5.56 mm weapons.
It has reportedly been combat-tested in Iraq in modified M16s with impressive results, although confirmation is hard to obtain."
"More recently, a challenger to the 6.8 mm has emerged in the form of the 6.5 mm Grendel from Alexander Arms.
This is an attempt to match the merits of the 6.8 mm while providing even better long-range performance good enough to completely replace the 7.62 x 51 mm.
The 6.8 mm Remington SPC is limited to relatively short bullets to fit within the maximum cartridge length of 5.56 mm weapons, so the Grendel has a shorter (39 mm) and fatter case, leaving room for longer, low-drag bullets, which retain their velocity out to long ranges.
Bullets of up to 9.3 g have been tested; with this loading the remaining velocity and energy at 1,000 m match the long-range 7.62 mm M118LR NATO round and are distinctly superior to the standard M80 7.62 mm ball. However, the optimum choice for general-purpose use appears to be a bullet of around 8 g, which matches the M80's trajectory. The 6.5 mm Grendel is at an earlier stage of development than the 6.8 mm Remington SPC, however, and has not been tested as extensively.
One of the penalties of switching to a more powerful round has already been mentioned: the recoil will be heavier. However, the recoil generated by the 6.8 mm or 6.5 mm is similar to that of the 7.62 x 39 mm AK-47 round, which has been successfully used in automatic weapons for over half a century, so it is clearly manageable.
Another is that the cartridge cases are slightly fatter. The 6.8 mm case is based on the old .30 Remington hunting round of 10.6 mm rather than 9.5 mm diameter, which reduces magazine capacity; a magazine similar in size to the standard 30-round 5.56 mm one will take about 28 rounds of 6.8 mm.
A third drawback is that the cartridge is heavier, so fewer rounds can be carried for a given weight, although the force of that argument is much reduced if it is necessary to fire more 5.56 mm rounds to achieve the same effect.
The 6.5 mm Grendel round is fatter still (it actually has the same 11.2 mm base diameter as the 7.62 x 39 mm), so only about 25 rounds will fit into a standard-sized magazine.
The ammunition is also slightly heavier than the 6.8 mm if heavy, long-range bullets are used. However, in the longer term it has more potential to replace the 7.62 x 51 mm and its weapons as well as the 5.56 x 45 mm family."
"What would be the penalties if the US (and presumably NATO at some point) were to decide to adopt a more powerful cartridge like the 6.8 mm Remington SPC or 6.5 mm Grendel?
Normally, such a change would be extremely expensive due to the investment in existing weapons and ammunition.
However, the US Army, joined probably by the other services, is proposing to replace all of its 5.56 mm small arms and does not have large stockpiles of 5.56 mm ammunition - indeed, they recently suffered a shortfall due to high expenditure in Iraq. There is therefore a window of opportunity to consider the replacement of the 5.56 x 45 mm cartridge before the new rifle competition is reopened, during which time it would seem sensible to carry out exhaustive tests of the 6.8 mm and 6.5 mm rounds against the 5.56 mm Mk 262 before reaching a conclusion about the future calibre.
One thing is certain: if the US eventually orders a new family of weapons, whichever cartridge is chosen will be in service for decades to come."
