Ammo lingo: Subsonic

In an ongoing effort to answer readers’ questions about ammunition types, this feature is on subsonic ammunition. If you’re looking for ballistic data to accompany the facts here, please look elsewhere; there are many places to find that. This is a layperson’s explanation, not a math project, which isn’t so easy to find unless you have friends in the ammo-making business.

What is subsonic ammo?

Subsonic ammunition is that which is made primarily for use with a suppressor, often called a “silencer.” It can also be used in a handgun or rifle all by itself, unsuppressed, to different effect.

When compared to other ammo within the same caliber, subsonic loads have a smaller powder charge inside the case, and generally a heavier bullet. In extraordinarily simple terms, we can think in terms of the formula mass times velocity equals force. When velocity is decreased by having less powder, and therefore less gas to drive the bullet down the barrel, through the air, and into its target -, a bullet of more mass compensates to a degree. For example, Atomic Ammunition’s subsonic load in .223 has a 75 grain bullet—not an extraordinary weight, but one associated with match rounds. An average .223 target match bullet weighs 55 grains.

It is the moment a bullet leaves the barrel, and the explosion of gas that’s behind it, that creates the “bang” of a firearm. Subsonic ammunition, traveling at lower than normal velocity relative to the caliber, is quieter. It still makes a bang that requires hearing protection when used sans suppressor. When in tandem with a suppressor, subsonic ammunition makes a hearing-safe pop, the kind you hear in the movies when the bad guy fires a gun with a silencer. (Funny, isn’t it, how it’s always a bad guy…more on our weird cultural beliefs on suppressed shooting in the follow-up to this article).

Why use subsonic?

Uses for subsonic are any application where less noise and/or less recoil are desired. Even when fired unsuppressed, it’s substantially quieter. That makes it a great choice for indoor or urban ranges, as well as hunting, especially for circumstances in which a landowner may want to eliminate more than a single varmint or pest animal. When fired using a suppressor, subsonic is less likely to scare away the rest of the herd. Some hunters claim the remaining animals may still spook, but since subsonic offers no muzzle flash and no directional bearing on sound, they may actually run in the hunter’s direction.

It’s also a good choice for teaching gun handling and marksmanship fundamentals to a new or very young shooter without the complication of recoil.

In addition, subsonic ammo helps to protect the irreplaceable asset of hearing. Ear protection is still required on non-suppressed firearms, but it’s still much quieter. Quieter shooting is an asset not just for gun users, but also for non-shooting neighbors who are less likely to object to our hobby—at home as well as in the voting booth.

To me, hearing protection and wider acceptance of our right to bear and use firearms are the biggest advantages, however long-term their realization.

The downsides

Some subsonic loads can, consistently or occasionally depending on the firearm, fail to dependably cycle the action. Though I had great fun shooting Atomic’s .223 load through my AR, being quite accurate at 175 yards while experiencing virtually no recoil nor roof-shaking noise, I had to cycle the bolt by hand.

A notable exception is the popular .300 AAC Blackout caliber, purpose-made for suppressed shooting. With a bolt carrier group and barrel change, it can be fired through the AR15 platform. This widely available load offers the AR owner great versatility from one firearm, although many feel it’s unnecessary unless it’s used with a suppressor.

Subsonic ammunition is a bit less accurate at longer distances. According to Atomic Ammunition rep Jerod Johnson of Arizona, the smaller doses of powder in subsonic loads can shift around within the case, producing less reliable flight. I experienced this in a field trial of the brand’s .308 caliber subsonic. In several three-shot groups, two rounds would be remarkably accurate; their impact holes touching. A third would be a modest flyer, three to five inches away from the others. It’s not a huge difference for most applications except where absolute precision is required. According to Johnson, subsonic rifle loads such as the .308 I tested at 100 yards are rather ineffective beyond 300 yards, where velocity loss is rapid.

The price of subsonic is, like match ammunition, reflective of the specialized manufacturing process. Expect to spend double or more the price of FMJ.

Get some

If there’s no admonishment against subsonic ammunition in your firearm’s user manual, trying out a box is a great experience, whether accompanied by a suppressor or not. There’s a surprising ease to firing more powerful rounds than a rimfire cartridge, while getting sound and recoil that are closer to the rimfire range. Try some for yourself.

All photos by Team HB.