Subsonic vs Supersonic Ammo: What the Speed of Sound Means for Your Bullets

The 1,125 FPS Threshold

The speed of sound at 68°F and sea level is approximately 1,126 fps (343 meters per second). The firearms community rounds this to 1,125 fps as a working reference. Any bullet traveling below this speed is subsonic; anything above it is supersonic.

But the speed of sound isn't fixed. It changes with temperature:

This matters in practice. A load running 1,070 fps at a summer range day in Arizona is comfortably subsonic. Take that same ammo to a January morning in Montana (0°F) and the speed of sound drops to 1,051 fps — your "subsonic" ammo might crack through the sound barrier and produce the supersonic snap you were trying to avoid.

Why Subsonic Matters: The Suppressor Connection

A suppressor reduces the muzzle blast — the explosion of expanding gas as the bullet exits the barrel. But a supersonic bullet also generates a sonic boom (the "crack" you hear downrange). A suppressor can't do anything about the sonic boom because it happens along the entire flight path, not at the muzzle.

This is why suppressed shooting has two tiers of quiet:

Suppressor + supersonic ammo: The muzzle blast is reduced, but you still hear a sharp, distinct crack. Think of it as going from "jet engine" to "car door slamming." Hearing protection is still recommended. The sound is roughly 130–140 dB depending on caliber and suppressor.

Suppressor + subsonic ammo: Both the muzzle blast and the sonic crack are eliminated. The sound drops to 110–120 dB — comparable to a car horn or a nail gun. With good suppressors and subsonic loads, some setups drop below 115 dB, which is hearing-safe for brief exposure. This is the "Hollywood quiet" setup, though it's never truly silent.

Common Calibers: What's Subsonic and What Isn't

The .300 Blackout: Purpose-Built for This

.300 Blackout was developed by Advanced Armament Corporation (AAC) in cooperation with Remington Defense, with R&D directed by Robert Silvers. SAAMI approved it on January 17, 2011. It was designed from the ground up to do two things from the AR-15 platform: run supersonic loads with ballistics comparable to 7.62x39, and run subsonic loads that are hearing-safe when suppressed.

The subsonic 220gr .300 BLK loads run 1,010–1,050 fps depending on barrel length, producing approximately 518 ft-lbs of muzzle energy — comparable to a .45 ACP +P. That's a meaningful step up from subsonic 9mm (roughly 300–350 ft-lbs) in a rifle platform that takes standard AR-15 magazines and lower receivers.

The Transonic Problem: Why Accuracy Falls Apart

The transonic zone sits between roughly Mach 0.8 and Mach 1.2 — approximately 900 to 1,350 fps. When a bullet decelerates through this zone, the shockwave attached to its nose detaches and reforms erratically. These shifting pressure waves push on the bullet asymmetrically, destabilizing the spin and causing the point of impact to wander.

This is called transonic wobble, and it's why long-range shooters care deeply about whether their bullet is still supersonic at the target distance.

For a 5.56 M193 at 3,100 fps muzzle velocity, the bullet doesn't hit the transonic zone until roughly 500–600 yards. For .308 168gr match loads, it's around 800–900 yards. For 6.5 Creedmoor 140gr ELD-M (BC .646), the high ballistic coefficient keeps the bullet supersonic past 1,200 yards — one of the main reasons 6.5 CM dominates long-range competition.

Subsonic ammo never enters the transonic zone because it starts below the sound barrier and only slows further. This means subsonic loads are inherently stable in terms of shockwave behavior — but they trade this for a dramatically curved trajectory due to their low velocity. A subsonic .300 BLK 220gr round drops approximately 38 inches at 200 yards. Precision past 100–150 yards requires significant holdover or dialing.

Subsonic Tradeoffs You Should Know

Less Energy

Kinetic energy is ½mv² — velocity is squared, so cutting velocity in half cuts energy by 75%. Subsonic 9mm 147gr at 1,000 fps generates roughly 326 ft-lbs. Supersonic 9mm 124gr at 1,150 fps generates roughly 364 ft-lbs. The gap isn't huge in 9mm, but in rifle calibers the difference is enormous: supersonic .300 BLK 125gr at 2,250 fps produces ~1,400 ft-lbs vs. subsonic 220gr at 1,050 fps producing ~518 ft-lbs.

JHP Expansion Can Suffer

Hollow point bullets rely on hydraulic pressure (fluid forcing open the cavity) to expand. At subsonic velocities, some JHP designs don't generate enough pressure to open reliably. The 9mm 147gr HST is a notable exception — Federal engineered it for subsonic expansion, and it achieves 0.606" diameter at roughly 1,000 fps. But not all JHP loads are designed for this. If you're running subsonic defensive ammo, verify gel test data at actual subsonic velocities from your barrel length.

Cycling Reliability

Semi-automatic actions need enough gas pressure to cycle the bolt. Very slow subsonic loads — especially in .22 LR — may not generate enough energy. CCI Quiet-22 40gr at 710 fps will not cycle a stock Ruger 10/22. The CCI Quiet-22 Semi-Auto 45gr at 835 fps will. If you're running a suppressed semi-auto, test your specific ammo for reliable cycling before depending on it.

The Bottom Line