Barrel length influences muzzle velocity because it determines how long the gases produced by the powder's combustion can exert pressure on the base of the bullet. Generally speaking, a longer barrel allows for longer gas expansion, while a shorter barrel stops this acceleration phase sooner.
The relationship between length and velocity, however, is not linear. There is no universal number of meters per second gained or lost for each centimeter of barrel: the result depends on the caliber, ammunition, type of powder, bullet weight, pressure developed, and the characteristics of the individual weapon.
How is barrel length measured?
In breech-loading weapons, the barrel length is generally measured from the face of the closed bolt to the tip of the muzzle. In semi-automatic weapons and bolt-action rifles, the chamber is therefore included in the overall measurement.
Any muzzle brakes, flash hiders or other devices attached to the muzzle are not normally part of the barrel unless they are integral or permanently attached in accordance with the criteria established by the applicable regulations.
As the bullet accelerates in the barrel
After the primer strikes, the powder begins to burn, producing gases at high temperatures and pressure. When the force exerted on the base of the projectile overcomes the initial resistance, the projectile begins to move and engages the rifling.
As the projectile advances, the available volume behind it increases. The pressure reaches a maximum in the initial stroke and subsequently decreases, but continues to push the projectile as long as the force produced by the gases remains greater than the resistance provided by friction and the air in the barrel.
A longer barrel can therefore offer greater acceleration room. The advantage gradually diminishes because, as you move towards the muzzle, the residual pressure becomes increasingly lower.
Why a longer barrel can increase speed
If the bullet leaves the muzzle while the gases still have significant pressure, a longer barrel length can allow more of the energy of the gases to be converted into velocity.
Profit depends mainly on:
- quantity and characteristics of the powder;
- liveliness and progressiveness of the propellant;
- internal capacity of the case;
- weight and contact surface of the projectile;
- residual pressure along the barrel;
- core diameter;
- friction between bullet and rifling;
- gas tightness;
- temperature of the ammunition and the barrel.
Yield decreases as length increases
Each additional centimeter of barrel doesn't necessarily produce the same increase in velocity. In the first few centimeters, the bullet is subjected to high pressures and accelerates rapidly. Further down the barrel, the pressure decreases, and the contribution of each additional centimeter generally becomes smaller.
We are therefore talking about diminishing returns : beyond a certain length, a longer barrel can offer a modest gain in velocity compared to the increase in weight and bulk of the weapon.
Can a barrel that is too long slow down the bullet?
Theoretically, if the barrel were long enough, the residual pressure could become so low that it could no longer compensate for the friction between the bullet and the rifling. From that point, the bullet would stop accelerating and could begin to lose velocity.
In standard rifle lengths, this point is rarely reached with the ammunition for which the gun was designed. However, it is possible that, beyond a certain range, the gain becomes negligible, or that some particular combinations show no further appreciable gains.
Effects of shortening the barrel
By shortening a barrel, the bullet exits earlier and has a shorter gap to exploit the gas pressure. Muzzle velocity therefore tends to decrease, but the amount of loss varies greatly from cartridge to cartridge.
Shortening can also cause:
- higher residual pressure at the muzzle;
- increase in perceived noise;
- more evident sprint flame;
- greater effect of gases on muzzle brakes and other devices;
- reduction of the kinetic energy of the projectile;
- modification of trajectory and fall at various distances;
- possible variation of operation in semi-automatic weapons.
There is no standard loss per centimeter
It's common to find average values ​​for velocity loss per centimeter or inch of barrel removed. These figures can be useful for a preliminary assessment, but they're not a universal rule.
Loss can be limited within a certain length range and become more pronounced as the barrel is further shortened. Two cartridges of the same caliber, but loaded with different powders or bullets, can also react differently to the same change in length.
Bright powders and progressive powders
Relatively lively powders produce gas quickly and can reach their full potential in shorter barrels. More progressive powders, on the other hand, maintain gas production for a longer period and can benefit from a longer barrel.
However, complete combustion shouldn't be equated with the end of acceleration. Even when most of the powder has reacted, the gases already produced continue to expand and can exert a useful thrust on the projectile.
Case capacity and relationship to caliber
Cartridges with a large case capacity relative to bullet diameter often use large amounts of relatively progressive powder. These rounds tend to require longer barrels to effectively exploit their velocity potential.
Cartridges with a smaller capacity and greater internal efficiency can instead retain a greater part of their performance even with relatively short barrels.
Bullet weight and barrel length
The weight of the bullet affects the relationship between pressure, acceleration, and residence time in the barrel. A heavier bullet generally accelerates more slowly and remains in the barrel longer, while a lighter one can quickly reach high velocities.
However, the effect of length depends on more than just weight. The contact surface, jacket material, diameter, projectile shape, and the type of propellant used in the ammunition also play a role.
Barrel length in rifles
In rifles, the choice of length represents a compromise between ballistic performance, handling, weight and intended use.
A long rod can offer:
- greater speed with cartridges using progressive powders;
- lower pressure and flame at the muzzle;
- increased distance between the sights in iron sights;
- greater stability when shooting from a fixed position.
A short barrel can instead guarantee:
- more compact and easy to handle weapon;
- greater ease of movement in the woods or from restricted positions;
- reduced overall weight;
- faster aiming point movements.
The velocity advantage of a long barrel must therefore be evaluated together with the actual needs of use.
Barrel length in handguns
In pistols and revolvers, even small differences in length can affect velocity because the path available for acceleration is much shorter than that of a rifle.
Ammunition intended for handguns generally uses relatively high-velocity propellants. A longer barrel can, however, increase velocity, reduce residual muzzle pressure, and reduce the muzzle flash.
In revolvers, gas leakage through the space between the cylinder and the forcing cone must also be considered, a phenomenon which is absent in semi-automatic pistols with a chamber integrated into the barrel.
Barrel length in smoothbore shotguns
In smoothbore shotguns, the influence of length on shot velocity is generally less pronounced than is often believed. The powders used in shotshells develop the necessary pressure quickly, and most of the acceleration occurs in the initial stroke.
A longer barrel may offer a moderate increase in velocity, but its main advantages often involve:
- regularity of the weapon's movement;
- better swing controllability;
- wider line of sight;
- weight distribution;
- reduction of the shooter's perception of noise and flash.
Barrel length alone doesn't determine the spread. Choke, cartridge characteristics, wad, shot size, and barrel quality all play a more direct role.
Does length affect accuracy?
A longer barrel does not automatically mean more accurate barrels. Accuracy depends primarily on the quality of the workmanship, the uniformity of the bore, the chamber, the throat, the crown, the ammunition, and the rigidity of the system.
Given the same outside diameter, a short barrel is generally stiffer and may exhibit lower amplitude vibrations. A long barrel, however, offers greater velocity and can reduce flight time and wind influence at long distances.
Length also affects the barrel's harmonics , or how the tube vibrates during firing. Two barrels of different lengths can therefore exhibit different behavior even when using the same ammunition.
Speed ​​and kinetic energy
The velocity directly influences the kinetic energy of the projectile, expressed by the relationship:
Energy = ½ × mass × velocity²
Since velocity is squared, a change in velocity can produce a proportionally greater change in energy. The reduction in velocity due to a shorter barrel must also be evaluated in relation to the bullet's terminal behavior and the range of use.
Influence on the trajectory
A higher initial velocity tends to produce a flatter trajectory. For the same projectile and atmospheric conditions, the flight time is reduced and the gravitational pull at the same distance is reduced.
A lower speed generally results in:
- longer flight time;
- greater bullet drop;
- greater exposure to wind action;
- reduction of residual energy;
- possible change in expansive behavior.
Any significant change in barrel length therefore requires a re-check of the zeroing and ballistic data.
Declared speed and real speed
The velocities quoted by ammunition manufacturers are obtained using specific test barrels and controlled conditions. The barrel length used in the test may differ from that of the actual firearm used by the shooter.
Actual speed may also vary as a result of:
- actual chamber and barrel dimensions;
- manufacturing tolerances;
- ammunition temperature;
- loading batch;
- condition and cleanliness of the barrel;
- distance of the chronograph from the muzzle.
Two weapons of the same model and with barrels of the same length can therefore provide slightly different velocities.
Measuring speed with a chronograph
The most reliable method for determining the velocity developed by a specific weapon-ammunition combination is to use a ballistic chronograph . The test should include multiple shots to obtain an average velocity and evaluate the ammunition's consistency.
When comparing barrels or ammunition, it's important to keep the temperature, barrel condition, and gun configuration as constant as possible. The data obtained with different guns cannot be attributed solely to length, as individual barrel tolerances also play a role.
The optimal length of the barrel
There is no optimal length that applies to all calibers and applications. The choice must take into account:
- cartridge features;
- weight and type of bullet;
- required speed;
- usual shooting distance;
- type of hunting or sporting discipline;
- need for transport and handling;
- weight and balance of the weapon;
- noise and flame at the muzzle.
A long barrel generally allows for better use of large-capacity cartridges and progressive-action powders. A short barrel favors compactness and rapid use, accepting a possible reduction in velocity.
A relationship that depends on the entire system
Barrel length cannot be evaluated in isolation. Velocity and performance depend on the interaction between the case, propellant, projectile, pressure, friction, and the firearm's construction.
Generally speaking, increasing length increases the time during which the gases can accelerate the bullet, but the advantage becomes progressively smaller. The most effective choice does not necessarily coincide with the barrel that produces the maximum velocity, but rather with the one that offers the best balance between ballistic performance, accuracy, weight, and handling.
