Understanding Subsonic Versus Supersonic Ammunition

Jason Baird

Jason Baird

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Dr. Bomb explains gunshot noise and how it relates to understanding subsonic versus supersonic ammunition. Stay tuned as he further delves into how suppressors affect ammo.

I imagine many of you grimace while watching TV shows and movies where villains are dressed in black running around shooting suppressed firearms that are highly effective against everything (and everyone) they shoot.

Of course, according to video and film productions, non-military or police use of “silencers” is only for bad guys who silently kill people and then get away because no one hears the single gunshot that kills. Seems like the only changeup from this we see is a “double tap” in movies trying to appeal to watchers who have some video game knowledge of shooting, but the noise is still only a “pffffft – pffffft.” We know better.

Nevertheless, the main reason most of us invest the time and money to purchase and obtain a suppressor to use with pistols and/or long guns is to reduce the firing and downrange noise of gunshots. We realize we’ll likely never achieve the Hollywood “pffffft,” but what should we expect, noise-wise?

Dr Bomb on the range with SilencerCo Switchback 22
Dr Bomb on the range in Utah with the SilencerCo Switchback 22 (Barbara Baird photo)

The Reason for Gunshot Noise

Bear with me as I take you through a simplified explanation of gunshot noise sources. It may help you make informed decisions about gun, suppressor and ammo purchases. In the process, I’ll focus on the biggest factor in noise, ammunition, but briefly mention the other factors regarding caliber and suppressors along the way.

A shooter must consider other factors in addition to suppressor type and performance.

Ammunition choices, especially subsonic versus supersonic, play a huge part in the amount of noise suppression possible.

I’ll use the military-sounding term weapon system because you must wrap your brain around the fact that this is what we are assembling when we make choices regarding a firearm + caliber + suppressor + ammunition.

As you may guess, I’ll lay out some of the basic facts regarding the performance of subsonic ammunition fired through a suppressed weapon system. Choosing the ammunition to use, based on its projectile velocity when fired in your weapon, is an important step, because a suppressor by itself does not affect all noise from the shot, and noise reduction is a big reason to purchase a suppressor.

Think about it – what noises result from firing a weapon system (besides the “oohs” and “ahs” from folks watching your Instagram-post-able shooting technique and cool gun accessories)?

The Sequence of Gunshot Noise and the Supersonic Factor

  1. Starting from the butt-end of the system, we have the sound made by the action as it chambers, fires, extracts and re-chambers a cartridge. This happens unless you use a bolt-action or other single-shot firearm. So, one way to reduce firing noise is to use a single-shot firearm.
  2. Next, there is the sound of the explosion in the gun chamber as the cartridge fires. If you fire a large weapon system you might hear a distinct “ping,” but in small arms very little of that noise comes through the steel chamber walls.
  3. The portion of that noise you usually hear is what comes out of the muzzle, and it is combined with the noise created by the heat and turbulence of combustion gases expanding into the air. This noise we try to suppress by installing a suppressor on the muzzle.
  4. Another noise is the sound of the bullet hitting the target, and we cannot do anything to modify that aside from shooting a different target.
  5. The last noise is the sound a bullet makes after it leaves the firearm, if it speeds along at or above the speed of sound in the air (where most bullets travel). A supersonic bullet creates shock waves that spread out along the bullet’s path very similar to the shock waves created by supersonic aircraft and missiles; nearly everyone has heard the resulting “sonic booms.” The shadowgraph photo shows these shocks from projectiles. Most shooters never hear the shocks because gunshot noise overwhelms the “crack” of a sonic boom from a bullet, a sound reminiscent of someone firing a cap gun (small caliber gun, say .22 LR) or slapping a wet flip-flop against a hard surface (large caliber gun).
Bullet leaving the muzzle
Bullet, followed by pieces of bullet metal, leaving the muzzle. The turbulent, energetic gas is evident as the white cloud, with still-burning particles of gunpowder spraying from the barrel. (Herra KuulaPaa photo, Caters News)
Projectile shadowgraph for subsonic versus supersonic ammunition article
Shadowgraph of projectiles traveling through air at about 1.5 times sonic velocity. (A. Davidhazy photo, Rochester Institute of Technology)

Subsonic Versus Supersonic Ammunition Info

It happens that we can affect the noise of these sonic booms by firing subsonic ammunition, which eliminates the shock waves. The medium (usually air) through which the projectile is fired and the medium’s density (depends on temperature, humidity, and altitude) determine the local sonic velocity.

You might correctly guess that it takes an involved computation using the results of several measurements to arrive at a precise sonic velocity, but a good rule of thumb is that the local sonic velocity is about 1100 to 1150 feet per second (fps) under the conditions most of us shoot (all the way up to 9,000 feet under normal conditions).

Given this information, a shooter can consult the muzzle velocity information given by most ammunition manufacturers to decide whether particular ammunition is likely to produce a sonic “snap” when fired through a suppressed weapon.

 

Cartridge lineup rifle ammo subsonic versus supersonic ammunition
L to R: 3 x .300 AAC Blackout: 203 grain coated practice round, subsonic, handload; 212 grain plastic tipped hunting round, subsonic, handload; 155 grain match round, supersonic, commercial. 5.56 x 45 mm NATO Ball 62 grain, supersonic, commercial. 2 x 7.62 x 51 mm NATO: M80 Ball 147 grain, supersonic, commercial; M61 AP 150 grain, supersonic, handload. .30 – ’06 M2 AP 165 grain, supersonic, handload

In some cases, the internal ballistics of the cartridge may make this impossible. Unless you are a handloader, finding subsonic ammunition in popular centerfire calibers is difficult. Even handloading subsonic ammo in many calibers is problematic, if not outright dangerous, so if extreme quiet is of supreme importance, this should influence your choice of firearm caliber.

For more information about this particular problem, do some background research on the genesis of the .300 AAC Blackout caliber. If you do an online search, you’ll find several hours’ worth of reading on the topic.

Ruger American Ranch Rifle .300 AAC Blackout with Harvester silencer
PSA PA-15 Multi carbine
PSA PA-15 Pistol

Another Benefit of Using a Suppressor

Ammunition has an effect on suppressed firearm systems’ performance in a different way other than noise, and in this case being suppressed may improve the performance with subsonic ammo. When firing subsonic ammo using a suppressor on semi-auto .22 LR caliber handguns and rifles or pistol-caliber carbines, the systems are often less sensitive to ammunition quality than when you fire them without a suppressor. Many of these semi-autos are blowback-operated firearms, in many cases making them finicky consumers of ammo. Suppressing these firearms increases the operating pressure, allowing them to use lower pressure (i.e. subsonic) ammo that normally is hit-or-miss in the unsuppressed gun when it comes to misfeeds and stovepipes. You may also find that suppressing your larger caliber gas-operated pistol- or carbine-length AR causes operating problems because of the higher operating pressure, unless you use subsonic ammo.

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