Of interest this is taken from a 1986 study commissioned by the US Army of the M16A2. Elsewhere in the study, it states that independent testing done by the NRA showed the M16A1 with M193 ammo to be more accurate at 200mtrs then the A2/M855 combo. The Army was primarily concerned with accuracy to 300mtrs.
It is almost 30 y/o data, and I'd be interested in knowing if any of it has been superseded or disproved by scientific testing.
"While any alternative must be tested, previous firing tests have confirmed that a 1:9 twist will provide for stability of a bullet similar to the new ball round. Reducing barrel twist to 1:9 will result in less stress on the bullet, barrel life will be improved, and barrel fouling will be reduced. While this twist may not fully stabilize the tracer round to maximum range, the rifle tracer is normally used as a marking round at extended ranges and precise accuracy is not required.
The M16A1 has one twist (rifling) for each 12 inches of barrel length and it has very effective terminal ballistics against personnel targets. It is generally accepted that less bullet stability will enhance terminal ballistics. Therefore, the increased twist of the M16A2, one twist in seven inches, should be tested against a one in nine twist barrel, which would probably produce better terminal ballistics against personnel targets.
A very important consideration is that reducing twist to 1:9 will probably improve accuracy at all ranges, particularly at 25 meters and in the primary range band out to 300 meters. A general rule is that minimum twist should be used to stablilize the round. Any additional twist will increase variability, causing the bullet to move in a corkscrew-type pattern at closer ranges. With the Army dependence on 25-meter ranges, this factor is much more important to the Army than the Marine Corps. An example of appropriate twist may be seen in the Army Marksmanship Unit rifles which have a 1:10 twist for firing at ranges up to 1000 yards. When the same type rifle and ammunition is used for 300-meter competition, the twist is reduced from 1:10 to 1:14 to obtain greater accuracy. From reviewing available firing data and giving consideration to terminal ballistics, employment ranges, barrel fouling, accuracy, and compatibility with M193 ammunition, a 1:9 twist appears optimum for the Army rifle."