What is the difference between subsonic and supersonic ammo?
Supersonic ammunition leaves the barrel faster than the speed of sound; subsonic ammunition stays below it. The speed of sound in air is about 1,125 fps at 68°F at sea level and falls in colder air. A supersonic bullet produces a small shock wave along its flight path — the "crack" heard downrange. A subsonic bullet does not.
That difference matters mainly to suppressor users. A suppressor reduces the muzzle blast, but it cannot touch the crack a supersonic bullet makes after it has left the muzzle. Subsonic ammunition through a suppressor removes that second source of noise, which is why the two are usually discussed together. Without a suppressor, the muzzle blast is far louder than the bullet's crack, so subsonic ammunition makes little practical difference to what the shooter hears.
How fast is the speed of sound for ammunition?
The speed of sound depends mainly on air temperature (humidity and altitude have much smaller effects). Calculated from the standard formula for dry air:
| Air temperature | Speed of sound |
|---|---|
| 32°F (0°C) | about 1,087 fps |
| 59°F (15°C) | about 1,116 fps |
| 68°F (20°C) | about 1,126 fps |
| 95°F (35°C) | about 1,155 fps |
That works out to roughly 1 fps per degree Fahrenheit. The practical consequence: a load that is subsonic on a summer day can cross into supersonic on a cold one, and a load published near the line (for example, 1,050–1,100 fps) leaves little margin. Manufacturers that market loads as subsonic generally keep their published velocity well below the line for that reason.
What makes a gunshot loud?
A gunshot produces two separate sounds:
Muzzle blast. Hot, high-pressure gas leaving the barrel behind the bullet. This is what a suppressor reduces, by giving the gas room to expand and cool before it exits.
Sonic crack. The shock wave from a supersonic bullet, generated continuously along its flight path. A suppressor does nothing to it, because it forms outside the muzzle.
A third, smaller source is the firearm's action cycling. Measured sound levels depend on the specific firearm, suppressor, ammunition and measurement setup, so comparisons are only meaningful when those are held constant. Suppressor manufacturers publish their own measurements for named host firearms and loads.
Which calibers have subsonic options?
| Caliber | Subsonic status | Notes |
|---|---|---|
| .22 LR | Standard-velocity loads are subsonic | CCI lists its Standard Velocity load at 1,070 fps; high-velocity loads such as Mini-Mag are listed at 1,235 fps |
| 9mm | 147gr loads are usually subsonic from pistols | Federal lists its 147gr loads at 1,000 fps from a 4-inch barrel; 115gr and 124gr are supersonic |
| .45 ACP | Standard 230gr loads are subsonic | Published at well under 1,000 fps from standard pistol barrels |
| .300 Blackout | Designed with subsonic loads in mind | Subsonic loads use heavy bullets, roughly 190–220gr, at around 1,000–1,050 fps |
| .308 Winchester | Specialty subsonic loads exist | Heavy bullets at around 1,050 fps or below; most of the cartridge's range and energy is given up |
| 5.56 NATO | Rare | The cartridge depends on velocity for its intended terminal behavior |
A longer barrel adds velocity, so a load that is subsonic from a pistol can be supersonic from a carbine. Check the published velocity for your barrel length, or chronograph the load.
What do you give up with subsonic ammo?
Slower bullets carry less kinetic energy (energy rises with the square of velocity), and many bullet designs need a minimum impact velocity to expand or fragment. How much that matters depends on the caliber:
9mm 147gr. Many duty-class 147gr hollow points (Federal HST and Speer Gold Dot among them) are designed to expand at subsonic velocity, and manufacturers publish FBI-protocol results for them. See 9mm concealed carry loads compared.
.300 Blackout subsonic. Heavy, slow .30-caliber bullets behave more like large pistol bullets: they do not fragment, and expansion depends entirely on bullet design. Some subsonic loads use bullets engineered to expand at those speeds. Supersonic .300 Blackout loads (110–125gr) are a separate category with very different terminal behavior. See .300 BLK subsonic loads compared.
.45 ACP. Already subsonic in standard loadings, so there is nothing to give up relative to its normal performance.
.308 Winchester subsonic. Turns a long-range rifle cartridge into a short-range, low-velocity one; it exists mainly for suppressed bolt-action use.
Is subsonic ammo useful without a suppressor?
Mostly not. Unsuppressed, the muzzle blast dominates what you hear, so the subsonic load gives up velocity and energy without a matching drop in noise. Hearing protection is still required.
The usual exception is .22 LR from a rifle: a small powder charge burned in a long barrel, combined with subsonic velocity, is noticeably quieter than high-velocity .22 LR. It is still not hearing-safe.
What problems can subsonic ammo cause?
Cycling in semi-automatic rifles. Subsonic loads produce less gas. In gas-operated rifles such as AR-15s, they may not cycle the action reliably without an adjustable gas block, a different buffer, or a host built for subsonic use. Bolt actions are unaffected.
Point of impact. Subsonic and supersonic loads strike at different points at most distances, so a rifle zeroed for one is not zeroed for the other.
Mixing in one magazine. Mixed velocities mean mixed points of impact and, in gas guns, possible cycling differences. Most shooters load one type per magazine and mark which is which.
The .300 Blackout / 5.56 mix-up. .300 Blackout rifles share magazines and lowers with 5.56 rifles, and a .300 Blackout cartridge can chamber in a 5.56 rifle — firing it can destroy the rifle. See .300 Blackout vs 5.56.
How much does subsonic ammo cost?
In most centerfire calibers, subsonic loads are specialty products made in smaller volumes than standard supersonic loads, and are usually priced above them per round. .22 LR is the exception, since standard-velocity loads are common. The gap moves with the market, so compare current tracked prices rather than a fixed figure:
.300 Blackout subsonic → | 9mm 147gr subsonic → | .45 ACP 230gr →
Tracked category pages: .300 Blackout subsonic, 9mm subsonic, .22 LR subsonic, and the Ammo Price Index for every caliber.
Related articles
- .300 BLK subsonic loads compared
- .300 Blackout vs 5.56
- 115gr vs 124gr vs 147gr 9mm
- What does grain mean?
Sources
- Speed of Sound — Wikipedia — Temperature dependence of the speed of sound in air
- Federal American Eagle 9mm Luger 147gr FMJ (AE9FP) — Published 147gr velocity
- CCI Rimfire Ammunition — Standard Velocity and Mini-Mag specifications
- SAAMI Technical Information — Industry standards