The Transonic Zone Explained: Why Subsonic .22 LR Isn’t Always Silent
If you’ve ever fired subsonic .22 LR through a suppressor and heard an unexpected sharp crack, you may have encountered the effects of a projectile operating close to the speed of sound. The label “subsonic” does not guarantee that every round will produce the same quiet report in every firearm or weather condition.
The important variables are actual velocity, temperature, ammunition consistency, barrel length, and the specific firearm. A suppressor can reduce the muzzle blast, but it cannot eliminate the aerodynamic sound produced by a projectile traveling at or near Mach 1. (silencerco.com)
What Does “Subsonic” Actually Mean?
Subsonic ammunition is ammunition whose projectile remains below the local speed of sound.
At approximately 59°F (15°C), the speed of sound in air is about 1,116 fps under standard atmospheric conditions. At 70°F, it is approximately 1,128 fps, while at 100°F it rises to roughly 1,159 fps. (
That means there isn’t one universal velocity that defines “subsonic” under every environmental condition.
The distinction matters with suppressed .22 LR because the suppressor can significantly reduce the sound of expanding gas at the muzzle, leaving the projectile’s aerodynamic signature much more noticeable. SilencerCo notes that supersonic ammunition continues to produce a crack even when fired through a suppressor. (silencerco.com)
Why the Speed of Sound Changes
The speed of sound in air is strongly affected by temperature.
As temperature increases, the speed of sound increases. At standard atmospheric pressure, representative values are approximately:
| Air Temperature | Speed of Sound |
|---|---|
| -40°F | 1,004 fps |
| 0°F | 1,051 fps |
| 32°F | 1,087 fps |
| 70°F | 1,128 fps |
| 100°F | 1,159 fps |
Altitude can also affect the speed of sound indirectly because atmospheric temperature changes with altitude. Under a standard atmosphere, the speed of sound decreases as temperature falls at higher elevations.
Humidity has a smaller effect than temperature, but it can also change the speed of sound.
For rimfire shooters, temperature is generally the environmental variable that matters most when considering whether a marginally subsonic load will remain below Mach 1.
What Is the Transonic Zone?
The transonic regime is the range of flow conditions around Mach 1.
NASA defines subsonic flow as Mach numbers below one, supersonic flow as above one, and transonic flow as the region around Mach 1 where compressibility effects become especially important. In transonic flow, portions of the airflow around an object can become locally supersonic even when the object’s overall speed remains below Mach 1.
That distinction is important for understanding projectiles.
A bullet does not have to travel faster than the ambient speed of sound for the airflow immediately around parts of its surface to reach locally supersonic conditions. As the projectile approaches Mach 1, compressibility effects and shock-wave formation become increasingly important.
However, it is too simplistic to describe a fixed velocity band such as “850 to 1,340 fps” as the universal transonic zone for every .22 LR bullet. The exact flow behavior depends on projectile shape, Mach number, atmospheric conditions, and other aerodynamic factors.
Why a Subsonic .22 LR Can Still Sound Sharp
The most common explanation is simpler than many shooters realize: the ammunition may not actually be traveling as slowly as the box suggests in that particular firearm and environment.
Manufacturers publish velocity figures from specified test barrels, so your rifle or pistol may produce a different result. SilencerCo notes that .22 LR subsonic ammunition is commonly designed around approximately 1,050 fps, but actual velocity can change with barrel length. (silencerco.com)
If a load is close to the local speed of sound, small changes in velocity or temperature can move it closer to or above Mach 1.
There is also a second factor: the projectile’s local flow field becomes increasingly compressible near Mach 1. That can contribute to aerodynamic noise before the bullet’s measured average velocity clearly exceeds the ambient speed of sound.
So hearing a crack does not necessarily mean the ammunition label is wrong. It may mean that your particular setup is operating much closer to the sonic boundary than expected.
Why Ammunition Consistency Matters
Two rounds from the same box do not necessarily leave the muzzle at exactly the same velocity.
Shooters and ammunition manufacturers use standard deviation (SD) and extreme spread (ES) to describe velocity consistency.
For suppressed rimfire shooting, consistency matters because ammunition that averages only slightly below the local speed of sound leaves less margin for individual shots to approach or cross the threshold.
A load averaging comfortably below the local sonic threshold generally provides a larger margin than ammunition whose advertised velocity sits close to it.
That is one reason experienced suppressed-rimfire shooters often prefer loads with predictable velocity rather than choosing solely from the lowest advertised number.
Barrel Length Changes the Equation
The advertised velocity on a .22 LR box is not necessarily the velocity your firearm will produce.
Ammunition manufacturers commonly publish rimfire velocity from a specified test barrel. Your actual barrel can be shorter or longer, producing a different velocity.
SilencerCo specifically notes that many .22 LR subsonic loads are measured from approximately 16-inch barrels and that velocity can fall significantly when the same ammunition is fired through a short pistol barrel. (silencerco.com)
That means the same ammunition can behave differently in:
- A 16-inch rimfire rifle
- A 10/22-type carbine
- A short-barreled rifle
- A rimfire pistol
This is one of the biggest reasons box velocity should be treated as a reference rather than a guarantee.
Temperature and Subsonic .22 LR
Temperature can make a marginal difference surprisingly noticeable.
Consider a hypothetical load producing around 1,100 fps. At 32°F, the speed of sound is approximately 1,087 fps, putting that projectile above the local sonic threshold. At 70°F, the speed of sound is around 1,128 fps, putting the same projectile below it. (engineeringtoolbox.com)
That illustrates why environmental conditions matter.
A load that behaves as subsonic at one temperature can behave differently at another, particularly when its actual velocity is close to Mach 1.
Subsonic vs. Standard-Velocity .22 LR
The terminology can get confusing because “standard velocity,” “subsonic,” and “high velocity” aren’t always defined around identical thresholds by every manufacturer.
For suppressed shooting, the key question is not what category appears on the box. It is whether the projectile remains below the local speed of sound in the actual firearm.
SilencerCo recommends considering both ammunition velocity and the firearm when evaluating suppressed rimfire performance, noting that barrel length can substantially change actual velocity. (silencerco.com)
Why Some .22 LR Loads Are Quieter
A suppressed rimfire shot contains several separate sounds.
There is the mechanical action, the discharge and expanding gas at the muzzle, and potentially the projectile’s aerodynamic shock-wave signature.
A suppressor primarily reduces the gas-related component. It cannot remove the aerodynamic sound generated farther downrange by a projectile traveling supersonically. (silencerco.com)
That’s why the difference between a suppressed high-velocity .22 LR and a suppressed subsonic load can be immediately obvious.
The subsonic load eliminates the dominant ballistic crack when it remains sufficiently below Mach 1.
Why “Subsonic” Does Not Mean Silent
This is perhaps the most important distinction.
Subsonic does not mean silent.
Even a genuinely subsonic .22 LR still produces:
- Muzzle blast
- Mechanical noise from the firearm
- Projectile impact noise
- Sound from the action cycling on a semi-automatic firearm
A suppressor can reduce the muzzle report, but it does not eliminate every sound produced by the firearm and projectile.
SilencerCo’s testing and educational material makes the same distinction: a suppressor reduces sound, but ammunition traveling above the speed of sound still produces a noticeable crack. Practical Considerations for Suppressed .22 LR
If quiet performance is your priority, the most useful approach is to think in terms of margin rather than labels.
Choose ammunition designed to remain comfortably below the expected local speed of sound, rather than selecting a load that sits just below a nominal 1,125-fps threshold.
Then test that ammunition in the specific firearm you intend to use.
A chronograph can provide useful information about actual muzzle velocity, while listening to the firearm outdoors can help identify whether a particular ammunition-and-firearm combination is producing an obvious ballistic crack.
The exact result will still change with temperature and environmental conditions.
What Should You Look For in Subsonic .22 LR?
For suppressed use, the most useful characteristics are:
Low actual velocity: The farther below the local sound barrier, the more margin you have.
Consistent velocity: Lower shot-to-shot variation reduces the likelihood that occasional rounds approach the sonic boundary.
Reliable cycling: A very low-velocity cartridge may not provide enough energy to cycle every semi-automatic rimfire firearm. SilencerCo specifically notes that some subsonic .22 LR ammunition can produce cycling problems in certain platforms.
Good accuracy in your firearm: Every barrel can have its own ammunition preferences.
There is no single subsonic .22 LR load that is automatically ideal for every firearm.
Frequently Asked Questions
Why does my suppressed .22 LR still crack with subsonic ammunition?
The ammunition may be operating close to the local speed of sound, particularly if actual velocity is higher than the manufacturer’s test-barrel figure. Temperature, barrel length, and ammunition variation can all affect the result.
What is the speed of sound in fps?
At 59°F, it is roughly 1,116 fps under standard atmospheric conditions. At 70°F it is approximately 1,128 fps, and at 100°F approximately 1,159 fps.
Does temperature affect subsonic ammunition?
Yes. The speed of sound increases as air temperature rises. Ammunition operating near the threshold can therefore behave differently under different weather conditions.
Does barrel length affect .22 LR velocity?
Yes. The velocity printed on an ammunition box is generally obtained using a specified test barrel. A shorter or longer barrel can produce different actual velocity. SilencerCo specifically notes that subsonic .22 LR can lose velocity when fired from short-barreled pistols compared with longer test barrels.
Is subsonic .22 LR completely silent with a suppressor?
No. A suppressor reduces muzzle blast but cannot eliminate mechanical noise, impact noise, or aerodynamic sound completely. The absence of a sonic crack is one part of making a suppressed .22 LR quieter, not the entire equation.
Does every subsonic .22 LR work reliably in a semi-automatic firearm?
No. Some very low-velocity ammunition may not generate enough energy to cycle every semi-automatic rimfire action. Function can vary significantly between firearms.
Bottom Line
The transonic zone helps explain why suppressed .22 LR can be more complicated than simply buying a box labeled “subsonic.”
The speed of sound changes with temperature, ammunition velocity varies from shot to shot, and the actual velocity produced by a particular firearm can differ substantially from a manufacturer’s test-barrel figure. Near Mach 1, aerodynamic effects become increasingly important, and portions of the airflow around a projectile can become locally supersonic even when the projectile’s overall speed is still below Mach 1. (nasa.gov)
The practical lesson is simple: subsonic ammunition is about staying sufficiently below the local speed of sound, not merely carrying a subsonic label.
For the quietest suppressed .22 LR setup, choose a load with a meaningful velocity margin, test it in the actual firearm, and account for temperature and barrel length.
That approach will tell you considerably more than the number printed on the ammunition box.