
Projectile differences.
The increase in projectile length, weight, and configuration of the
M855 bullet requires different twists in the barrels, lands, and
grooves to stabilize the bullet in flight. The M16A1 has a 1:12
barrel twist (the bullet rotates once for every 12 inches of travel
down the barrel). The M16A2/A3/A4 and the M4 carbine has a 1:7
barrel twist (the bullet rotates once for every 7 inches of travel
down the barrel).
The M16A1, with its 1:12 twist, does not put enough spin on the
heavier M855 bullet to stabilize it in flight, causing erratic
performance and inaccuracy for training or full combat usage
(30.48- to 35.56-centimeter shot group at 91.4 meters and 72-inch
shot group at 274.2 meters).
Although firing the M855 cartridge in the M16A1 rifle is safe, it
should only be used in a combat emergency, and then only for close
ranges of 91.4 meters or less.
Full article read here:
http://www.jimwardrip.com/firearms/556_Ballistics.htm