In the early 1980s the M855’s 62-grain bullet was developed for the
M249 Squad Automatic Weapon (SAW). For purposes of interoperability, the
same load was adopted as the M16A2 rifle’s standard ball as well. A
February 1986 U.S. Army study noted that the M855’s bullet required a
"1:9 twist [which] would be more appropriate for the M16A2 rifle,
improving accuracy and reliability.” Multiple studies confirmed the
1:9-inch twist requirement.
   
But then a problem arose. The U.S. military’s standard M856 5.56 mm
tracer round was longer, heavier (63.7 grains) and slower than the M855
ball, and simply would not stabilize with a 1:9-inch twist barrel. Thus,
despite it doubling M855 group sizes, the M16A2 (and later, the M4)
specified a 1:7-inch rate-of-twist barrel to stabilize the tracer round.
It remains so to this day.