300 Blackout subsonic is the reason the cartridge exists. A heavy 190 to 245 gr .30-caliber bullet at about 1,000 to 1,050 fps stays below the speed of sound, makes no supersonic crack, and still carries about 450 to 480 ft-lb at the muzzle. Paired with a suppressor in a short barrel, that makes a compact, quiet firearm for hunting, training and defense. Chattahoochee Munitions manufactures 300 Blackout subsonic firearms on the CM7S bolt action in 8, 10 and 12.5 inches with matching barreled actions, and on the CM15 AR15 in 5, 7.5, 10.5 and 16-inch barrels, the CM15-SD and CM15 uppers, all with 1:5 twist barrels except the 1:7 16-inch.
300 Blackout Subsonic at a Glance
| Chattahoochee Munitions recommendation | |
|---|---|
| Bullet weight | 190 to 245 gr |
| Muzzle velocity | About 1,000 to 1,050 fps (factory loads) |
| Barrel length | 10 to 10.5 inches for general use; 5, 7.5 and 8 inches for the smallest package |
| Twist | 1:5 |
| Suppressor | Enclosed, on bolt action and semi-automatic |
| CM15 gas | Pistol length (micro length on the 5-inch), adjustable gas block, adjustable buffer |
| Practical hunting range | Inside about 100 yards |
What Is 300 Blackout Subsonic Ammunition?
Subsonic ammunition keeps the bullet below the local speed of sound, which is about 1,125 fps at sea level on a mild day and lower in cold air. Factory 300 Blackout subsonic loads leave margin under that line. They use long, heavy bullets so the bullet keeps its weight, and its energy, at a low speed. Hornady publishes its 190 gr Sub-X at 1,050 fps and 465 ft-lb and its 208 gr ELD Match subsonic at 1,020 fps and 480 ft-lb, both from a 16-inch barrel. Cartridge and chamber dimensions follow the ANSI/SAAMI centerfire rifle standard.
300 Blackout Subsonic vs Supersonic
| Subsonic | Supersonic | |
|---|---|---|
| Bullet weight | 190 to 245 gr | 110 to 150 gr |
| Muzzle velocity | About 1,000 to 1,050 fps | About 2,000 to 2,400 fps |
| Muzzle energy | About 450 to 480 ft-lb | About 1,300 to 1,400 ft-lb |
| Drop at 200 yd | About 34 in (100 yd zero) | 0 (200 yd zero) |
| Supersonic crack | None | Yes |
| Suppressor on a CM15 | Enclosed | Flow-through |
| Practical hunting range | Inside about 100 yd | Inside about 200 yd |
Supersonic 300 Blackout carries about three times the energy and shoots flatter. Subsonic gives up range for sound: with a suppressor, the shot is far quieter because nothing cracks downrange. Chattahoochee Munitions treats subsonic as the cartridge’s main job, because other cartridges do supersonic work at longer range. Changing between the two moves point of impact by many inches and changes how a CM15 cycles. See 300 Blackout ballistics for full tables.
Barrel Length for 300 Blackout Subsonic
In Chattahoochee Munitions’ experience the 300 Blackout’s powder burn is effectively complete in about 9 to 10.5 inches, and a longer barrel adds no useful subsonic velocity. That is why CM recommends 10 to 10.5 inches for general use. The 5 and 7.5-inch CM15s and the 8-inch CM7S give up a little velocity for a shorter suppressed firearm. The 16-inch CM15 is for owners who want a rifle outside the NFA, not for more subsonic speed. See 300 Blackout barrel length.
Twist Rate for Heavy Subsonic Bullets
A subsonic bullet spins about a third as fast as a supersonic bullet from the same barrel, and it is longer. Both call for a faster twist. Chattahoochee Munitions uses 1:5 on every CM7S barrel and on the 5, 7.5 and 10.5-inch CM15, which keeps long 220 to 245 gr bullets stable even in cold air. The 1:5 barrels are designed for subsonic loads, not supersonic. The 16-inch CM15 uses 1:7 and is the CM 300 Blackout for supersonic ammunition. See 300 Blackout twist rate.
300 Blackout Subsonic Is Not Silent
A subsonic bullet removes the supersonic crack. The suppressor contains muzzle blast, but some sound escapes, and the bullet still makes noise on impact. A semi-automatic adds the sound of its bolt carrier cycling. A bolt action stays closed when it fires, so with an enclosed suppressor the CM7S leaves only the suppressed report. Two firearms firing the same subsonic load through the same suppressor can sound and run differently.
Bolt Action or Semi-Automatic for Subsonic
The CM7S bolt action has no gas system. Any subsonic load, light or heavy, feeds and ejects by hand, and changing loads changes only point of impact. It uses a Mos-Tek 416R stainless 1:5 barrel, a TriggerTech R700 Primary trigger and a 10-round CM7S 300 Blackout bolt action magazine.
The CM15 semi-automatic cycles on gas, and subsonic loads make little of it. The CM15 300 Blackout uses pistol-length gas on the 7.5, 10.5 and 16-inch CM15 and micro-length gas on the 5-inch, with a Superlative Arms adjustable gas block and an ODIN Works adjustable buffer, and feeds from the CM15 300 Blackout magazine. Every complete CM15 is test-fired and set to a baseline before it ships. A standalone CM15 upper is tuned on the owner’s lower. See 300 Blackout bolt action vs semi-auto.
Gas First, Buffer Second
On a CM15, set the gas block to the least gas that feeds, ejects and locks the bolt back on the last round, then refine with the buffer. A heavier buffer slows the carrier but does not remove extra gas. Even two subsonic loads can need different settings. The 300 Blackout gas system guide gives Chattahoochee Munitions’ single-round tuning procedure and the 3:00 to 4:30 ejection window.
Suppressors for 300 Blackout Subsonic
Use an enclosed suppressor. On a bolt action it gives the sound reduction. On a semi-automatic its back pressure also adds energy to cycle a low-pressure subsonic load, where a flow-through design can leave the action short of gas. With supersonic ammunition in the 16-inch CM15, change to a flow-through suppressor. Every Chattahoochee Munitions 300 Blackout barrel is threaded 5/8×24. See the 300 Blackout suppressor guide.
SD: The Suppressor Under the Handguard
On a Chattahoochee Munitions CM15, SD means part of the suppressor sits inside an enlarged handguard. The firearm is shorter with the suppressor on, and the suppressor stays a separate, NFA-regulated item; the firearm is not integrally suppressed. CM offers the 5-inch 300 Blackout PDW, the 7.5-inch 300 Blackout tucked suppressor and the 10.5-inch 300 Blackout SD.
Choosing 300 Blackout Subsonic Ammunition
Choose by job, not bullet weight alone. A match bullet can be accurate at 1,000 fps and not expand. A hollow point designed for rifle speed may not open either. For hunting, choose a bullet designed to expand at subsonic speed, such as a solid-copper expanding design. Black Butterfly Ammunition offers subsonic loads from 200 gr Maker expanding copper to 220 gr and 245 gr target loads. Chattahoochee Munitions does not load 300 Blackout; it loads niche cartridges such as 308 Winchester subsonic. See 300 Blackout hunting.
300 Blackout and 5.56 Safety
A 300 Blackout cartridge can chamber in a 5.56 rifle. Firing it can destroy the firearm and injure the shooter. Use dedicated, marked magazines, store the two cartridges apart and check the headstamp before loading. See What Is 300 Blackout? for the safety details.
300 Blackout Subsonic Firearms From Chattahoochee Munitions
| Priority | Firearm |
|---|---|
| No action noise, any subsonic load | CM7S 300 Blackout bolt action SBR, 10-inch |
| Short suppressed bolt gun | CM7S 300 Blackout truck gun, 8-inch |
| Hunting with walnut furniture | CM7S 300 Blackout hunting rifle, 12.5-inch, with WOOX walnut furniture |
| General-purpose semi-automatic | CM15 300 Blackout AR15 SBR, 10.5-inch |
| Compact semi-automatic | CM15 300 Blackout CQB, 7.5-inch |
| Smallest semi-automatic | CM15 300 Blackout backpack gun, 5-inch |
| Suppressor under the handguard | CM15-SD 300 Blackout SD, 10.5-inch |
| Rifle outside the NFA | CM15 300 Blackout AR15 rifle, 16-inch |
| Upper for your lower | CM15 300 Blackout upper, 10.5-inch |
| Barreled action for your chassis | CM7S 300 Blackout barreled action, 10-inch |
See every configuration in Sector 300. For heavier subsonic bullets, see 338 ARC vs 300 Blackout and 8.6 Blackout vs 300 Blackout.
300 Blackout Subsonic FAQ
What velocity is 300 Blackout subsonic?
Factory loads run about 1,000 to 1,050 fps, below the speed of sound of about 1,125 fps at sea level on a mild day.
What barrel length is right for 300 Blackout subsonic?
Chattahoochee Munitions recommends 10 to 10.5 inches with a 1:5 twist for general use.
Is 300 Blackout subsonic good for hunting?
For hogs and deer inside about 100 yards with a bullet designed to expand at subsonic speed, yes, where legal.
Will my AR15 cycle 300 Blackout subsonic?
It needs the right gas system. A short gas system and an adjustable gas block, as on every CM15, let it be tuned for subsonic loads.
Is subsonic 300 Blackout quiet without a suppressor?
Quieter downrange because there is no crack, but the muzzle blast is still loud. Hearing protection is required without a suppressor.
Should I use an enclosed or flow-through suppressor for subsonic?
Enclosed, on both bolt actions and semi-automatics.
Can one gas setting run every subsonic load?
Not always. Confirm function each time you change the load or suppressor.
What gas system does a subsonic 300 Blackout AR15 use?
Every CM15 300 Blackout uses an adjustable gas block and an adjustable buffer, with pistol-length gas from 7.5 to 16 inches and micro-length gas on the 5-inch, so the owner can set it for the subsonic load and suppressor.
Who makes a 300 Blackout subsonic rifle?
Chattahoochee Munitions manufactures 300 Blackout firearms for subsonic use: the CM7S bolt action in 8, 10 and 12.5 inches with barreled actions, and the CM15 AR15 in 5, 7.5, 10.5 and 16-inch barrels, the CM15-SD and CM15 uppers.