Subsonic ammunition is ammunition designed to operate at speeds below the speed of sound, which at standard conditions is 343.2 m/s (1,126 ft/s). 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.
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 7.5 grams (116 gr) bullet at velocities typically around 400 metres per second (1,300 ft/s). Subsonic loads for 9mm commonly use 9.5 grams (147 gr) bullets at velocities of 300 metres per second (980 ft/s).
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 military standard .45 ACP ammunition load, of a 230 grain bullet, is 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 / 300 AAC Blackout (7.62×35mm), .338 Whisper, 9×39mm, 12.7×55mm STs-130, .510 Whisper.
Use with suppressors
Combined with firearm sound suppressors, subsonic ammunition may significantly reduce sound levels compared to normal ammunition. Specific reductions depend on the ammunition and suppressor.
- Paulson, Alan C (1996). Silencer History and Performance, Vol 1: Sporting And Tactical Silencers. Paladin Press. ISBN 0-87364-909-5.
- Paulson, Alan C; Kokalis, Peter G.; Parker, N.R. (2002). Silencer History and Performance, Vol 2: CQB, Assault Rifle, and Sniper Technology. Paladin Press. ISBN 1-58160-323-1.