In a news article today the reporter claims that modern improvements in "ammunition design" have made long rife barrels unnecessary for assault weapons. Here is the paragraph:
The 20-inch barrel isn’t needed anymore.
One of the biggest differences between the M4 and the M16A4 is the barrel length: 14.5 inches versus 20 inches, respectively. The M16A4’s longer barrel allowed for a higher muzzle velocity and a longer effective range; conversely the shorter M4 barrel limited its performance in both categories. But improvements in ammunition design have enabled the M4 to close the gap with its longer forefather.
The Marine Corps adopted a new 62-grain, 5.56×45 mm Special Operations Science and Technology round. The SOST round is designed to perform out of barrels as short as 10.5 inches, so the M4 has no difficulty shooting out to the extreme end of effective range, negating the advantage of the long M16A4 barrel.
What improvements in ammunition design are they referring to? Why would a bullet be equally accurate coming out of short barrel?