During the last decade there have been 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. Now there's a new round, the 300BLK. This is a 7.62 bullet using a similar size (35mm long) cartridge as the 5.56mm round. Thus, all you need is a new barrel for your M-4 or M-16 rifle. The larger and heavier 7.62mm round is 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. A year ago, the 300BLK was approved for manufacture, and tests both on and off the battlefield are under way. The 300BLK can use the same magazines as the 5.56mm round.
But the fate of the 6.8mm round should be considered before declaring the 300BLK has a bright future. Six years ago the new 6.8mm rifle round developed for SOCOM (Special Operations Command) 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.
The 6.8mm, 5.56mm, 300BLK, 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 that war, 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 (7.92mm for the Germans, 7.62mm for the Russians, which is still used in the AK-47).
This gave the infantryman an automatic weapon that could still fire fairly accurate 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.
SOCOM used the 6.8mm round in Iraq and Afghanistan and the troops liked it, but not enough to cause 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. At the moment, no decision has been made about any replacement for the 5.56mm round. The 300BLK is unlikely to change that.