Subsonic vs. Supersonic Rifle Ammunition: What You Need To Know

Whether hunting, target shooting, experimenting with handloads or simply just curious, you may be wondering: what’s the difference between subsonic and supersonic ammunition, and which is better for your rifle? This article will cover what you need to know when comparing high vs. low speed ammo, and when one might be a more suitable choice over the other.

In simple terms, subsonic means the bullet is moving slower than the speed of sound, while supersonic means the projectile breaks the sound barrier, producing a sonic crack as it travels through the air. While each have their own benefits, supersonic ammunition is most often the more responsible and ethical choice, specifically when related to hunting and defensive use. While subsonic ammo is quieter, producing less recoil, supersonic ammo produces a flatter trajectory, greater effective range and superior terminal performance (or stopping power) against living targets.

The majority of gun owners, whether they know it or not, are familiar with supersonic cartridges. These include the majority of pistol and rifle cartridges used for sports shooting, hunting and self defence. Subsonic loads have a more unique purpose, and are typically custom loaded by hand or purchased for a very specific reason which we will discuss in more detail below.


So What Is Supersonic Ammunition?

Supersonic ammo cartridges are designed to operate at velocities greater than the speed of sound, which causes the bullet to make a “crack” as it travels downrange. This is a result of the sonic boom which occurs when the bullet breaks the sound barrier.

These high speeds which operate above the speed of sound drastically increase the bullet’s velocity and effective range, flattening its trajectory and increasing stopping power.

Stopping power is the firearm’s ability to cause a target to be incapacitated or immobilised through use of a penetrating projectile

Supersonic ammunition uses advanced aerodynamical projectiles with the perfect weight and shape designed to travel at high velocities and often over long distances. Precision rifle ammunition will fall into this category.

When using a suppressor (or silencer) in conjunction with supersonic loads, the sound will still be obvious and audible, but the shot will make a dull thump rather than a loud bang, and locating the direction in which the shot originated from can be difficult. The supersonic crack however cannot be removed, and can be heard from quite a distance.


What is Subsonic Ammunition?

Subsonic cartridges are loaded to operate at speeds less than the speed of sound, which prevents the bullet from making a supersonic shockwave or “crack” as it travels through the air.

When used in conjunction with suppressors, the subsonic bullet significantly reduces the total sound signature of the firearm, as the majority of the muzzle blast is suppressed by the silencer, while the lower velocity projectile removes the supersonic crack. The only noise that remains are those of the mechanical moving parts made by the firearm, and a small amount of noise produced by the escaping gasses at the muzzle end.

The downside is that subsonic ammo reduces the effective range and stopping power of the firearm, while also incurring a much steeper trajectory. For this reason, subsonic ammunition usually uses heavier bullets that retain more energy and momentum, but are still most often suitable only for plinking use.

Rimfires are where subsonics really shine. While the energy to cleanly kill small game remains in this caliber, the downrange crack vanishes.

Field & Stream: The Pros and Cons of Subsonic Cartridges

Note: another downside to subsonics are that they fail to cycle the bolt (or BCG) in most semi-automatic rifles. For this reason they are generally preferred amongst users of bolt action rifles.


The Speed of Sound And Its Effect On Projectiles

The sound barrier, otherwise known as the sonic barrier, is the sudden increase in aerodynamic drag and other undesirable effects experienced by the projectile as it approaches the speed of sound.

  • In dry air at 20°C (68°F), the speed of sound is 343mps or 1125ftps.

As temperature changes, so will the sound barrier. But how? This is because heat, like sound, is a form of kinetic energy. Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly. Sound will therefore travel faster in hotter environments, and slower in cold environments.

Knowing the speed at which the sound barrier sits is useful in determining a rifle’s effective range, and also in keeping subsonic loads subsonic when still trying to achieve maximum stopping power but without going supersonic.


Transonic Stability: How Your Bullet Breaches The Sound Barrier

The transonic effect related to spin-stabilised projectiles deals with the critical period of disruptive transition as the supersonic bullet reaches speeds close to the sound barrier. In simpler terms, once parts of the airflow reach subsonic speeds, the projectile becomes unstable and bullet tumbling most often follows.

When spin-stabilised projectiles slow down to a point where they are reaching the sound barrier, the centre of pressure shifts forward, causing the bullet to pitch and yaw. At this point the bullet struggles to retain enough gyroscopic stability to overcome the increasing dynamic instability that is experienced. While some bullets will manage to overcome these effects better than others, it is very difficult to determine how well a bullet will transition through the transonic phase.

Once a bullet reaches its transonic stage, precision will be difficult to achieve and the projectile will most often deviate to a new, unpredictable flight path where tumbling may result. For this reason, marksmen normally restrict themselves to engaging targets within the supersonic range of the projectile used, which is known as the rifles effective range.


Subsonic vs. Supersonic: The Pros and Cons

SUBSONIC PROJECTILESSUPERSONIC PROJECTILES
Most Silent OperationLouder, Presenting a Supersonic Crack
Steeper Bullet DropFlatter Trajectory
Insufficient Stopper PowerSuperior Stopping Power & Terminal Performance
Minimal Effective RangeMaximum Effective Range
Slower Velocity & Reduced RecoilHigher Velocity
Target Plinking and Small Varmint or Pest ControlLong Range Target Shooting, Hunting and Self Defence

Summary

While there are a few calibers that have been designed specifically to accommodate both subsonic and supersonic loads in the same rifle, with minimal adjustments (such as the .22LR and .300 Blackout), the statement below applies to the majority of rifle cartridges.

Subsonic cartridges are fun to shoot, but there is little demand and even less benefits to using them. For this reason, factory loaded subsonic cartridges cost more than their supersonic counterparts, and they should only really be used for plinking.

If your setup revolves around hunting or self defence applications, stick to supersonic ammunition and you won’t go wrong.


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1 Response

  1. Pete says:

    I’ve never heard or read that subsonic .300 AAC Blackout would not cycle an AR action as is implied in this article. For self-defense inside a home, it would seem the lower velocity– still fatal to a human– would be less likely to escape into a neighbor’s home. Really, how much extra velocity do you need at twenty-five feet?

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