Last week I began our suppressor discussion by giving the reader a high level view of what a suppressor is, what it does and does not do, and what benefits accrue from using a suppressor. I said that the two most significant components of weapon related noise is the muzzle blast and ballistic crack, which you may recall is the noise made by the bullet flight as it breaks the sound barrier. I also mentioned that subsonic ammunition is often used to mitigate the risk of producing a ballistic crack. Additionally, I spoke about the possibility of subsonic ammunition generating a ballistic crack given the suppressor’s free bore boost and prevailing atmospheric conditions.
In this segment, I’d like to discuss subsonic ammunition in greater detail to provide the reader with a deeper understanding of the subject.
Two components contribute to a bullets lethality; they are the cross-sectional area of the bullet – larger calibers produce greater tissue damage – and kinetic energy, which is the energy of the projectile on target impact. Kinetic energy provides the penetration and fragmentation necessary to enhance effectiveness. The equation for kinetic energy is k= (m*v²)/2.
A full power .308 load leaves the muzzle at 2600 fps, and as it travels through its flight path it remains above the speed of sound until it has traveled a distance of approximately 1100 yards or more. Because it travels faster than sound, it produces a loud crack that can be detected easily by the human ear or sensors.
To mitigate the probability of detection, subsonic ammunition is sometimes used. Because the velocity of the round is reduced from 2600 fps to 1050 fps manufacturers increase the mass of the projectile; doing so will preserve the rounds terminal ballistics. Referring back to the kinetic energy formula we can readily see that when velocity is reduced kinetic energy is reduced; so, to maintain an effective level of kinetic energy we increase the projectile’s mass from 175 grains to 220 grains or 240 grains.
RUAG Swiss P manufactures extremely high quality subsonic ammunition. It is, by far, the best I’ve ever tested in terms of consistency and vertical spread across a wide range of ambient temperatures.
Using subsonic ammunition places additional requirements on both the operator and platform, so it needs to be approached cautiously.
Because a heavier projectile is used, the barrel’s rifling needs to be no slower than 1:11, and preferably 1:10. Using rifling that is slower than 1:11 will fail to stabilize the round and may result in a baffle strike. Should the bullet impact against the suppressor, or its baffle stack, injury to the shooter or those in the surrounding area could result.
In addition to rifling requirements, subsonic ammunition when used in semiautomatic weapons like the M110 SASS, SCAR Mk 17, or HK 417 will generally fail to cycle the action reliably. This is true for a gas piston operating system or direct gas impingement.
Finally, subsonic ammunition is designed for use in short-range engagements. Internet stories of 900 yard shots with subsonic ammunition are either showmanship or sea stories. Subsonic ammunition requires that the operator take up a firing position that is 300 meters or less from the target’s position.
In next week’s post, I talk about mounting technologies and discuss some of the pro’s and con’s.