Subsonic ammunition is
ammunition designed to operate at velocities below the
speed of sound
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
, which at standard conditions is or
Mach 1. This avoids the
supersonic shockwave or "crack" of a supersonic bullet, which, particularly for
suppressed firearms, influences the loudness of the shot.
Subsonic ammunition usually uses heavier bullets to retain as much energy as possible at the lower velocities. Some subsonic ammunition is used in non-suppressed firearms to gain the advantages of heavier bullet weights.
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Standard calibers
Subsonic versions of standard rounds
In this instance, heavier bullets are loaded in standard ammunition, which reduces muzzle velocity below the speed of sound.
As an example, the very common
9×19mm Parabellum standard military round is a bullet at velocities typically around . Subsonic loads for 9mm commonly use bullets at velocities of .
For these ammunition loads, balancing bullet weight and velocity are required to ensure that the ammunition will still reliably cycle semi-automatic firearms. Subsonic ammunition with normal bullet weights often fails to properly function in such firearms.
Inherently subsonic calibers
Some ammunition types were inherently designed with heavier, slower standard bullet weights and velocities. For example, the traditional American military standard
.45 ACP
The .45 ACP ( Automatic Colt Pistol) or .45 Auto (11.43×23mm) is a rimless straight-walled handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it ...
ammunition load, of a 230
grain
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legum ...
bullet at 850
feet per second The foot per second (plural feet per second) is a unit of both speed (scalar) and velocity (vector quantity, which includes direction). It expresses the distance in feet (ft) traveled or displaced, divided by the time in seconds (s). The correspond ...
, is naturally subsonic.
Specialized subsonic calibers
Alternatively, specialized firearms and ammunition may be used to optimize total subsonic ammunition effectiveness. These are designed from the start as dedicated subsonic projectile systems. Some examples include
.300 Whisper
The .300 Whisper is a CIP standard cartridge in the Whisper family, a group of cartridges developed in the early 1990s by J.D. Jones of SSK Industries. It was developed as a multi-purpose cartridge, capable of utilizing relatively lightweight ...
/
300 AAC Blackout (7.62×35mm),
.338 Whisper
The .338 Whisper is a wildcat cartridge
in the Whisper family, a group of cartridges developed in the early 1990s by J.D. Jones of SSK Industries. Unlike the smaller caliber cartridges in the Whisper family, loads for the .338 Whisper are mainl ...
,
9×39mm,
12.7×55mm STs-130,
.510 Whisper
The .510 Whisper is a subsonic rifle cartridge developed by SSK Industries for use in suppressed rifles. It is capable of firing a .51-caliber bullet weighing at roughly .
Overview
The .510 Whisper (13×47 mm) is the second of two 1/2 inch ca ...
.
Use with suppressors
[William J. Murphy, Gregory A. Flamme, Adam R. Campbell, Edward L.
Zechmann, Stephen M. Tasko, James E. Lankford, Deanna K. Meinke, Donald S. Finan & Michael
Stewart (2018) The reduction of gunshot noise and auditory risk through the use of firearm
suppressors and low-velocity ammunition, International Journal of Audiology, 57:sup1, S28-S41,
DOI: 10.1080/14992027.2017.1407459]
Combined with firearm sound suppressors, subsonic ammunition may significantly reduce sound levels compared to normal ammunition. Specific reductions depend on the ammunition and suppressor.
The peak sound pressure levels of a Remington 700 .223 caliber bolt-action rifle firing high-velocity (supersonic) ammunition using a GEMTECH G5-5.56 suppressor.
The peak sound pressure levels of a Remington 700 .223 caliber bolt-action rifle firing low-velocity (subsonic) ammunition using a GEMTECH G5-5.56 suppressor.
Compared to the high-velocity (supersonic) unsuppressed condition, the combination of subsonic low-velocity ammunition and a firearm suppressor provided a reductions of 45, 38, 40, and 48 dB, peak sound pressure level at the muzzle, left, right ears and behind the shooter, respectively.
References
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Ammunition