Barrel length greatly affects accuracy. When firing standard Nato 5.56 ammo a barrel length longer than 24 to 26 inches becomes impractical as it begins to negatively affect the rounds ballistics. However, the practical length for a 5.56 barrel is around 20 to 22 inches.
This stems from the fact that the original 5.56 ammo and the US M16s were designed for a 20 inch barrel. With that length the rifle is often capable of penetrating level 3 steel plates.
The downside of the longer barrels though is that the entire weapon tends to be longer as well. In the case of the original M16 the weapon was nearly 3.5 feet long. This length made maneuvering in buildings and in and out of vehicles difficult.
Because of this some slight accuracy was given up and a shorter barrel was selected giving birth to the M4 Carbine.
Why Does Barrel Length Affect Accuracy?
When a round of a specific weight is propelled by a specific charge it will accelerate to a maximum speed. Depending on factors such as the weight, amount of the charge and even air density you can determine what this speed is.
Barrel length affects the overall velocity as the round needs enough time in the barrel to reach this maximum velocity. Ideally this will occur just before the round exits the barrel. The speed at which the round exits is known as Muzzle Velocity.
If the barrel is too short then the round leaves the barrel before it has reached its maximum velocity and the charge quits applying its effect to the round as it dissipates to the side.
If instead the barrel is too long then once the round reaches its maximum velocity the effects of friction from the barrel will begin to slow it down.
Velocity tends to affect accuracy due to the fact that there is less drop and less effect of external factors such as wind on a round that is traveling at a higher velocity.
Longer barrels also tend to have some flex in them and can actually whip slightly when fired. This whipping effect is transmitted to the round and can cause accuracy issues if the barrel length is too long.
Considering These Factors What Is the Optimal Length for a 5.56 Round?
If you take into account the various weights and powder charges commonly used for 5.56 rounds the optimal length of a barrel is between 14 and 22 inches. You can go slightly shorter or slightly longer but you begin to see diminishing performance once you step outside of the 14 to 22 inch lengths.