For the last two years, Special Forces troops have been testing a 6.8mm M-16 and M-4 weapons. The purpose of this is to provide an assault rifle round that has better accuracy and penetration at longer ranges, and does more damage to people and material (like walls or doors) at short ranges. The 6.8mm round (6.8x43mm long) was selected because it allows an M-16 to be adapted to the new cartridge with minimal effort (new barrel and a few adjustments.) Existing 6.8mm cartridges are longer than the M-16 round (5.56x45), thus the special 43mm long 6.8mm (.27 caliber) cartridge being used in the tests. But the 6.8mm round is fatter than the 5.56 round, so the 30 round M-16 magazine only holds 25 6.8mm rounds. There is also talk that the 6.8 round has enough heft to work as a "whisper round" (a low velocity, slower than the speed of sound bullet that can be fired from a silenced rifle and makes very little noise.) Special Forces like this sort of thing, as it makes it easier to achieve surprise and not be detected. Note that, 70 years ago, when the U.S. Army was developing a new rifle (the semiautomatic M-1 Garand), the designer suggested using a 7mm round. He was overruled by generals who believed (wrongly, as it turned out) that a lighter round would not be successful on the battlefield. One rifle that lost out to the M-1 design proposed using a 5.56x45 round. So there is a sense of dj vu to all this.
