Subsonic vs supersonic ammo

The short answer

The speed of sound at sea level is roughly 1,125 fps. Any bullet traveling faster than that creates a small shockwave — the "crack" you hear downrange. Supersonic ammo is above that threshold. Subsonic ammo stays below it, eliminating the crack.

This matters almost exclusively for suppressor users. A suppressor traps and cools expanding gasses at the muzzle, reducing the "bang." But if the bullet itself is supersonic, the sonic crack still announces the shot. Subsonic ammo through a suppressor eliminates both noise sources — the muzzle blast and the sonic crack — making the combination dramatically quieter.

Without a suppressor, the difference between subsonic and supersonic is mostly academic for practical shooting.

How it works

A gunshot produces two distinct sounds:

Muzzle blast. Hot expanding gas exiting the barrel. This is what a suppressor reduces — typically by 20–35 decibels, roughly the difference between a jackhammer and a vacuum cleaner.

Sonic crack. A small shockwave created by the bullet breaking the sound barrier. A suppressor does nothing to reduce this. The crack is generated along the bullet's flight path, not at the muzzle, so it can't be trapped.

Suppressed supersonic ammo is still loud — roughly 130–140 dB depending on caliber. Suppressed subsonic ammo can drop to 115–125 dB, which is hearing-safe or close to it without ear protection.

Which calibers have subsonic options?

Some calibers are naturally subsonic. Others need purpose-loaded subsonic ammunition. And some are effectively always supersonic — their velocity can't drop below 1,125 fps and still function reliably.

Representative suppressed sound levels vary by host firearm and suppressor model. Typical readings with a quality silencer: .22 LR subsonic ~113 dB, 9mm 147gr subsonic ~125 dB, .300 BLK subsonic ~126 dB, .45 ACP standard ~130 dB, 9mm supersonic ~136 dB, and 5.56 NATO ~140 dB.

The terminal performance tradeoff

Slower bullets carry less kinetic energy. That's not opinion — it's physics. The tradeoff is real and caliber-dependent:

9mm subsonic (147gr). Still performs well defensively. Modern 147gr JHP designs like Federal HST and Speer Gold Dot expand reliably at subsonic velocities. The FBI protocol doesn't require supersonic velocities — it requires 12–18 inches of gel penetration with consistent expansion, which 147gr loads achieve. Compare 9mm defensive loads →

.300 Blackout subsonic (190–220gr). The tradeoff is real here. Heavy, slow .300 BLK bullets behave like pistol rounds — they don't fragment, they don't tumble reliably, and expansion depends entirely on bullet design. Subsonic .300 BLK trades terminal performance for noise reduction. For home defense, supersonic .300 BLK (110–125gr) is significantly more effective on target.

.45 ACP. Already subsonic at standard loading. The .45's larger diameter (.452") partially compensates for lower velocity — it makes a bigger hole even without expansion. This is why .45 ACP remains a viable defensive caliber despite being "slow."

.308 Winchester subsonic. Turns a powerful rifle cartridge into a heavy, expensive pistol-energy round. Subsonic .308 has niche uses (extreme noise reduction in bolt-action platforms) but gives up everything that makes .308 useful — range, energy, terminal performance.

Do you need a suppressor?

Subsonic ammo without a suppressor is pointless in most cases. The muzzle blast alone is still 140–160 dB — well above the threshold for hearing damage. You're giving up velocity and terminal performance for a sonic crack reduction that's imperceptible next to the unsuppressed report.

The one exception: .22 LR subsonic from a rifle with a long barrel. The combination of low powder charge, long barrel (which allows more gas expansion), and subsonic velocity makes unsuppressed subsonic .22 LR noticeably quieter than high-velocity .22 LR. It's still not hearing-safe, but it's a meaningful reduction.

Cost comparison

Subsonic ammo generally costs more than supersonic in the same caliber because production volumes are lower:

Check live prices across all calibers →

What to avoid

Subsonic ammo in semi-auto rifles without adjustable gas. Subsonic loads produce less gas pressure. In gas-operated semi-autos (AR-15s, AKs), this may not cycle the action reliably. If you're running subsonic .300 BLK in an AR platform, you'll likely need an adjustable gas block or a different buffer weight. Bolt-action rifles don't have this issue.

Subsonic 5.56 or .223. It exists, but it defeats the purpose of the cartridge. 5.56 relies on velocity for terminal effect — at subsonic speeds, a 55gr bullet has less energy than most pistol rounds and won't fragment or expand.

Mixing subsonic and supersonic in the same magazine. Different velocities produce different points of impact. More importantly, alternating gas pressure can cause cycling issues. Load magazines with one type only.

The bottom line

Subsonic ammo exists for one primary reason: suppressor use. If you run a suppressor, subsonic ammo makes the entire system dramatically quieter. If you don't run a suppressor, subsonic ammo gives you less velocity, less energy, and potentially less reliability with no meaningful noise benefit.

The best subsonic calibers are .45 ACP (naturally subsonic, no compromise), 9mm at 147gr (minimal defensive performance loss), and .300 Blackout (purpose-built for suppressed use but with real terminal performance tradeoffs).

Compare suppressor-ready ammo prices →

Related articles

• Best .300 BLK subsonic
• .300 Blackout vs 5.56
• What does grain mean?

Sources

• Speed of Sound — Wikipedia — Physics of the sound barrier
• SAAMI Technical Information — Industry velocity standards
• Hornady Ballistic Calculator — Trajectory comparison tool