Silencer Saturday #359: A Primer On Subsonic Ammunition

Recently, the murder of UnitedHealthcare CEO Brian Thompson has absolutely dominated the news cycle. A silencer featured prominently in that assassination and led to all sorts of wild conjecture about what exactly was used. Another related topic was subsonic ammunition, both whether it was used and whether it would have accounted for the malfunctions seen with the suspect’s gun. This article is going to break down subsonic ammunition, what it is and what it isn’t, and why many of the popular assumptions from video games and the media are not very accurate.

Silencer Saturday @ TFB:

Silencer Saturday #358: A Holiday Suppressed Gift Guide Silencer Saturday #357: Black Friday Suppressor Sales Silencer Saturday #356: Reflex Silencer Roundup Silencer Saturday #355: More “DIY-Lisle” Adventures Silencer Saturday #354: SHOT Show 2025 Predictions

The Murder

I previously wrote about the murder of Brian Thompson and some of my observations from the now-imfamous security camera footage. The video led to all sorts of spurious assumptions and explanations about what happened. I also went so far as to do a demo with a Glock 19 and Dead Air Ghost silencer fitted with a fixed barrel spacer to show that a regular pistol without a functional Nielsen Device would look just like what we saw on the security camera video.

This event has had the public talking about assassinations and silencers in a huge way. I even found myself describing silencer mechanics to non-gun people at my day job because everyone was so curious about what had happened.

Many commentators, both informed and uninformed, offered opinions on the killer’s potential use of subsonic ammunition. Those takes ranged from accurate to wildly uninformed. Today we’re going to break it all down, and separate fact from fiction.

What Is Subsonic Ammo?

Everyone knows that bullets move quickly through the air. The phrase “faster than a speeding bullet” is an example of how common that knowledge is. But how fast are bullets, actually? That depends very much on what kind of bullet we are discussing. The speed of sound is usually about 1,100 feet per second, but it can be faster or slower depending on factors like temperature or humidity.

As a general rule, most rifle calibers tend to push a bullet at supersonic speeds, meaning that the bullet moves faster than the speed of sound. Most pistol calibers are subsonic, meaning they move slower than the speed of sound. If you are on the receiving end of gunfire, the bullet will reach you before you hear the sound of the shot if the bullet is supersonic. If the bullet is subsonic the reverse is true, you will hear the shot before the round impacts. There are many exceptions to these general patterns (e.g. 5.7×28 pistols are mostly supersonic, .300 Blackout rifles frequently use subsonic ammo) but that is the correct starting place.

But that is not the only difference. Just like a fighter jet going supersonic, bullets that break the sound barrier also create a sonic boom. The actual sonic boom is hard to identify when it happens at the same time as a gunshot. However, once a silencer is involved, the sonic boom tends to be the loudest part of the shot. While shooting suppressed it is very obvious if a round is supersonic or subsonic.

Let’s take a closer look at some sub and super ammo options.

Rimfire

.22 Long Rifle is one of, if not the, most prevalent calibers, and it is easily the most popular rimfire caliber. There are many different bullet weights and velocities available, some supersonic, and some subsonic. As another oversimplification, 40-grain bullets can be supers or subs, heavier bullets tend to be subsonic, and lighter bullets tend to be supersonic. Many manufacturers put the predicted velocity on the box, and some susbonic ammo also has branding that suggests it is suppressor optimized as an additional hint.

More niche rimfire options like .22 WMR and .17 HMR tend to be almost exclusively supersonic. But some, like .22 Short, are subsonic. Once again, rimfire ammo tends to include velocity figures on the box so as long as you remember that 1,100 FPS is the magic number you can tell which is which.

Another factor to consider is barrel length. Long barrels tend to increase velocity, and ammunition that is subsonic in a pistol may be supersonic in a rifle. And with a short enough barrel almost any ammunition will be subsonic, which is why some shooters take pistols like a Ruger Mk IV and chop the barrel back to an inch or two for dedicated suppressor use.

Rifle Ammo

Most centerfire rifle ammunition is supersonic. Much of the additional power of rifles over handguns comes from additional velocity. Most rifle cartridges have velocities of at least 2,200 FPS, with some as high as 3,000 or 4,000 FPS. That is great if you are trying to hunt large animals or hit targets at long range, but it is less than ideal for subsonic use.

Most of the conceptions about subsonic ammunition causing malfunctions are from rifle use. Taking a gun like an AR15 that is designed to operate using the gas created from a 3,000 FPS cartridge and using ammunition at under 1,100 FPS will cause many issues. There are a few specialized cartridges like the .300 Blackout and .338 ARC designed for subsonic use. The .300 Blackout is decently popular and it can use bullets as light as 110 grains for supersonic and as heavy as 265 grains for subsonic. Guns chambered for these cartridges can usually cycle both supers and subs.

Handgun Ammo

With handguns, things work differently. Many cartridges and loadings are subsonic because they simply cannot push a bullet fast enough to break the sound barrier. Others, like the common 9mm, can be subsonic or supersonic depending on the weight of the bullet. But the key distinction compared to rifle ammunition is that subsonic handgun ammunition is often more reliable than supersonic ammo. That may sound counterintuitive but because supersonic handgun ammo is usually within a few hundred FPS of the sound barrier, unlike rifles which are usually at least 1,000 FPS faster. That is much less of an energy difference and it presents less of a reliability issue.

Also, the heavier bullets used in subsonic pistol ammo can actually increase reliability. Many handguns work on the short recoil principle as pioneered by John Moses Browning. Rather than porting off the expanding gasses like most rifles do, handguns harness the recoil force of the shot to cycle the action. Moving to a heavier, but slightly slower, bullet often helps address cycling issues, which is why it is a common diagnostic step when a pistol won’t cycle reliably. So the general rule is reversed from rifle subsonic ammo, where it often causes more issues.

Many older suppressed handguns were chambered in .32 ACP or .380 ACP for other reasons. Those calibers were almost always subsonic in the standard loading. They also generally used fixed barrels, which tend to be more reliable when suppressed. Once again, using subsonic ammo does not make those guns less powerful or less reliable than they would otherwise be, because supersonic ammo is not really an option.

This is all to make two main points, that in a handgun shooting subsonic ammo is not weird or unusual, and that it is unlikely to be the cause of malfunctions. Even some popular YouTube gun channels made uninformed statements about subsonic ammunition being hard to obtain or easy for law enforcement to track. Depending on the caliber in question, that is often untrue. For example, a 9mm pistol was used to murder Brian Thompson, and subsonic 147-grain 9mm ammo is usually available at big-box retailers like Bass Pro or Sportsman’s Warehouse. It is not the most common ammo weight, but it does not require handloading or access to CIA weapons stashes. With other calibers, like .45 ACP, the most common ammunition has 230-grain bullets and is subsonic. Supersonic ammo does exist for that caliber but it is much less common.

In conclusion, subsonic ammo is not that special in many handgun or rimfire loadings. It is actually the default setting. But issues with subsonic loads in rifles, and stats decreases from equipping them in video games, has led to a public perception that these are oddball, unreliable loadings. That may be true in some use cases but it is far from the rule. Thanks for joining us for Silencer Saturday, we’ll see you next week.