Subsonic vs. Supersonic Ammo: The Differences You Need to Know

[Ammodotcom]

So you just saw a box of ammo labeled “subsonic” and another one labeled “supersonic” – both claiming to be for the same firearm.

What do those words mean with regard to ammo? What makes one different from the other? You must understand these things before you can make the most effective use of your firearm.

Fortunately, it’s not all that complicated. Today, we’re discussing subsonic vs. supersonic ammo: their differences, their relative pros and cons, and which one is better for your needs. By the end of this article, you’ll know which type of ammo you should choose.

Let’s get after it.

nderstanding the Speed of Sound and Its Effect on Ammunition

To understand the difference between subsonic and supersonic ammo, you must understand (A) the speed of sound and (B) how bullets are affected by it.

The speed at which sound waves travel depends on a few atmospheric variables (elevation, temperature, barometric pressure, humidity, etc.). For simplicity’s sake, it’s standard to assume the speed of sound equals 1,125 feet per second (ft/s; or 767 mph) at 68 °F. When an object exceeds the speed of sound (i.e., becomes supersonic), it breaks the sound barrier and issues a sharp cracking noise (sonic boom) as a result.

Ever been to an air show with fighter jets? Remember the awesome “ker-POW” as their engines fired off? Those were sonic booms.

What Is Supersonic Ammo?

Most modern ammo designs are supersonic by nature: i.e., loaded with bullets that exit the barrel at a velocity higher than 1,125 ft/s. Ignition of a supersonic cartridge is always accompanied by a distinctive “CRACK” – a literal sonic boom.

Supersonic bullets aren’t necessarily more powerful than subsonic alternatives (bullet weight also plays a significant role in striking energy), though they do tend to follow flatter trajectories to their targets. That facilitates aiming, as it allows the shooter to make less compensation for bullet drop (which becomes less pronounced when the bullet takes a relatively shorter amount of time to traverse a given distance).

What Is Subsonic Ammo?

Subsonic ammunition is designed to fire its projectiles at a velocity below the speed of sound (1,125 ft/s). It is typically loaded with heavier bullets than its supersonic counterparts (e.g., 300 BLK, which is supersonic at 150 grains and subsonic at 190).

Subsonic ammo’s performance is quieter than that of supersonic alternatives. It is unable to break the sound barrier, and therefore avoids producing a sonic boom on ignition.

I’ve always found subsonic ammo to have a softer report, generate less recoil, and deliver better long-distance accuracy. That third facet of performance is due to a quirk of physics. When a bullet’s velocity transitions from supersonic to subsonic, it undergoes what is known as “transonic destabilization.” This phenomenon causes the bullet to wander off of its intended trajectory — and is totally averted by a bullet that never becomes supersonic in the first place.

Continue reading Subsonic vs. Supersonic Ammo: The Differences You Need to Know on Ammo.com

https://ammo.com/comparison/subsonic-vs-supersonic-ammo#subsonic-vs-supersonic

[Devereaux]

Nice review.

One needs to emphasize the supersonic crack - it is important. IF your bullet remains reliably supersonic to target, then the bullet will hit with more energy. NOT necessarily more penetration just energy. THEN depending on bullet design you can get more wound channel disruption. 5.56 NATO is well known for this feature. Supersonic will also tend to fly farther and so long as it stays supersonic, be more accurate.

Just remember, these are games of numbers, design, distance, etc. HARD to beat the damage of a soft lead ball of sufficient weight and diameter. No fancy stuff, just old fashioned physics.

[Kilroy]

You nailed the supersonic crack point — that’s the part most people gloss over. The crack isn’t just noise; it’s the bullet still flying faster than sound can keep up, which tells you it’s carrying real energy downrange.

Once that bullet drops subsonic, you’ve lost the shock effect and the flight characteristics change. The 5.56 example is a good one.

That round depends on staying supersonic to do its damage — drop below the speed of sound and it becomes a completely different animal. And you’re right about the lead ball too.

Big, heavy, slow, and soft does a lot of work without any of the modern engineering. Old fashioned physics is hard to argue with.

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Kilroy was here

[Devereaux]

Perhaps I sort of misstated the supersonic crack issue.

Energy is … energy. E=1/2 MV2. So whether supersonic or subsonic, the energy is a constant curve. It diminishes smoothly with drop in velocity. BUT while the bullet is flying supersonically, it is stable. When it flies subsonically it is also stable. It is the “transition zone” that is the problem. The speed at whicch it drops from supersonic to subsonic. That is not a fixed number but varies with temperature, humidity, wind, altitude, etc. It does come in a “range”, Generally maintaining at least 1400 FPS at the target avoids the transition zone.

?What is the transition zone and why is it important. Well, it’s the speed where the bullet either first breaks into supersonic speed or out of same. That transition - from/to supersonic speed - causes the bullet to wobble. ONLY DURING the actual transition, but that could be for 100 yds for instance. THEN when it settles into subsonic speed, it is once more stable. HOWEVER, you really don’t know what trajectory the bullet will have ended up on while wobbling through transition. (The transition TO supersonic speed takes place inside the barrel so is of no consequence to bullet stability.)

SO, if you want a stable bullet flight, you want it to maintain supersonic speed all the way to the target. But DEADLY BULLETS have been known to be subsonic for a long time - .45-70 is an example of a primarily subsonic bullet. You fire it like artillery at range — and it kills quite reliably that way too. The original .45 ACP was only 800 FPS and it put down the Moro warriors (and Germans in WWI) quite reliably too.

[Kilroy]

You’re right — I had the transition zone backwards. The energy curve stays smooth; it’s the stability that gets ugly when the bullet crosses the sound barrier.

That wobble through the transonic region is the real problem. It’s not the bullet being supersonic or subsonic that hurts accuracy — it’s the crossing itself.

And you’re spot on that it’s not a fixed number. Temperature, humidity, altitude all shift where that transition happens, which is why a load that’s stable at 1500 fps in July might be fighting you in January.

The practical takeaway is what you said: keep it supersonic to the target, or commit to a subsonic load that never gets close to the barrier. Flirting with the transition zone is where groups open up.

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