If found this interesting though not relating to airsoft directly. The muzzle velocity of a 5.56mm round shot from a 14" to 16" M4 barrel is 2,700fps VS the standard M16A2 20" barrel which is 3,100fps. Wow that's a pretty big difference for only 4" to 6"!! Also does anyone know if the SS109 Isreali Armor Piercing round is the same as the Tungsten/Carbide Tip we employ in our M16/M4 series?
Yep. Muzzle velocity is somewhat proportional to barrel length, provided the identical bullet, in weight/shape/size, is used and the identical powder charge is used. Barrel length and thickness also increases accuracy. A longer barrel with a slower rate of twist in rifling provides a more stable flight. A thicker barrel reduces vibration and allows for less resonant distortion during flight. Then again, you probably already know some of this.