The U.S. Army has called on five manufacturers to deliver six competing designs for the new SAW (Squad Automatic Weapon, or light machine-gun) and M4 assault rifle replacements chambered for a new 6.8mm round. Current rifle and SAW designs in development or production can be offered as long as they accommodate new 6.8mm round. Prototypes will be delivered in 2021 and a winning design will be selected for low rate production a year or two after that.
Apparently, the 6.8mm round is also subject to further development and those firms willing to offer weapons prototype are expected to deliver a novel and practical 6.8mm cartridge design. The army is taking advantage of the substantial innovation in SAW and assault rifles design that has taken place since the 1990s. There has also been a lot of new ammo, of various calibers, designed. However, the new weapons and ammo may not displace the current weapons unless there are demonstrable improvements in performance without incurring loss of reliability. The new 6.8mm round may be caseless or have non-metallic (and lighter) cartridge case.
In some respects this is back to the future because in the 1930s the United States was developing a new combat rifle and weapons using both 6.8mm and 5.56mm rounds were evaluated. The winning American rifle design was the M1 Garand, which was originally designed to use the .270 Winchester cartridge. This was a 6.8mm round and was preferred because it was more accurate than the then standard 7.62mm 30-06 round. There were also proposals to use the 5.56mm 220 Swift (or similar rounds similar to the current 5.56mm rounds).
In the end, it was all about money, or the lack of it. The army wanted to get the Garand into mass production as soon as possible and it was noted that there was lots of World War I surplus 30-06 ammo available and that this was also the standard ammo for army rifles and light (the BAR) and medium machine-guns. So the 6.8mm and 5.56mm military rounds would have to wait for a chance to see what they could do in combat. The 5.56mm got its chance starting in the 1960s but it looks like the 6.8mm will end up waiting nearly a century.
There is some commonality here. The 6.8mm, 5.56mm, and 7.62x39 AK-47 round are all considered "assault rifle" rounds. This concept of a less powerful rifle round came out of research begun towards the end of World War I. During the 1930s, the Germans studied their World War I experience and concluded that a less powerful and lighter rifle round would be more effective. This resulted in research on a smaller 7mm round, but with World War II fast approaching this effort eventually produced a shortened regular (7.92mm) rifle round. During World War II the Germans developed the first modern assault rifle, the SG-44. This weapon looked a lot like the AK-47 and that was no accident. The SG-44, like the AK-47, used a shortened rifle cartridge that was developed before the war. These were 7.92mm for the Germans, 7.62mm for the Russians, which is still used in the AK-47, and after 2000 the similar 300BLK was an option for the M4.
Assault rifles gave the infantryman an automatic weapon that could still fire fairly accurate single shots at targets 100-200 meters away. The SG-44 and the AK-47 had about the same stopping power as the 6.8mm SPC and 300BLK at those shorter ranges. What a coincidence. The AK-47 didn't have the accuracy of higher powered bullets but the Russians didn't see this as a problem, because most troops using it had little marksmanship training. If they had to kill someone they could fire at full auto.
The U.S. M-16 and its high-speed 5.56mm round, was more accurate than the AK-47 when firing individual shots at shorter ranges. But the wounding power of the 5.56mm (.223 caliber) bullet fell off rapidly at ranges over a hundred meters. The American military, and especially SOCOM, train their troops to fire individual shots and do it with great accuracy at any range. A number of new rifle sights have made it even easier to do and makes first round hits at longer ranges easier to make. This made the longer range shortcomings of the 5.56mm round more obvious.
After 2001 there were several attempts to get the United States to replace the 5.56mm rifle round with something more powerful. A 6.8mm round was popular for a while but never caught on. There was even a new round for the M16, the 300BLK. This was a 7.62 bullet using a similar size (35mm long) cartridge as the 5.56mm round. Thus, all you needed was a new barrel for your M-4 or M-16 rifle. The larger and heavier 7.62mm round was more effective at blasting through walls and doors and many troops believe it has better stopping (of soldiers it hit) power. American troops would sometimes use captured AK-47s (and their 7.62mm ammo) to test this theory. This fed the demand for something like the 300BLK. In 2010, the 300BLK was approved for manufacture, and tests both on and off the battlefield were carried out. The 300BLK could use the same magazines as the 5.56mm round. The 300BLK was found to be useful for some SOCOM (Special Operations Command) operations but was not enough of an improvement to replace the 5.56mm round in general use.
The fate of the 6.8mm SPC round should be considered before declaring the new 6.8mm round has a bright future. In 2005 a 6.8mm rifle round developed for SOCOM became available commercially as the 6.8mm Remington SPC (Special Purpose Cartridge). There were some problems in manufacturing the 6.8mm SPC. Remington began work on the new round in 2002. It used the case from the old Remington .30-.30 (which was not a true .30-.30, as it was rimless). SOCOM began testing the 6.8mm round in M-16s and M-4s modified to accommodate it. The 6.8mm round was more accurate at longer ranges and had more hitting power than the 5.56mm round the M-16 was originally designed for. Out to about 600 meters the 6.8mm round had about the same impact as the heavier 7.62mm round used in sniper rifles and medium machine-guns. But the 6.8mm SPC was found to be less accurate than 5.56mm rounds in some situations and put more stress on some rifle components.
SOCOM used the 6.8mm SPC round in Iraq and Afghanistan and the troops liked it, but not enough to create a clamor for widespread adoption. There was also resistance from senior (non-SOCOM) generals to any consideration for replacing the 5.56mm round with the 6.8mm. To further complicate matters, there was a new 6.5mm “Grendel” round being tested as well and some troops preferred it to the 6.8mm SPC. This was because the 6.5mm round is more accurate than the 6.8mm one at ranges beyond 500 meters.
The army is depending on the many recent (since the 1990s) improvements in technology (for cartridge and bullet design as well as new propellants) will produce a 6.8mm round able to demonstrate sufficient superiority and reliability to current ammo to justify the switch. This effort will be aided by adopting new weapons designs, especially ones that have built on new tech successfully implemented in combat over the last decade or so.