300 Blackout, By the Numbers
300 Blackout was standardized by SAAMI in 2011 after Advanced Armament Corporation developed it from the .300 Whisper concept for one purpose: a .30-caliber cartridge that runs in a standard AR-15 bolt, magazine, and upper, burns its full charge in a very short barrel, and shoots heavy subsonic bullets quietly through a suppressor. It is the 5.56 case trimmed and necked up to .308, which is why only the barrel changes and why a 300 BLK round will chamber in a 5.56 barrel with catastrophic results. The three rings behind the muzzle threads on every Faxon 300 BLK barrel exist because of that.
The cartridge lives two lives. Subsonic loads, 190 to 240+ gr at about 1,050 fps, are the reason it exists: suppressed, they are as quiet as a centerfire rifle gets. Supersonic loads, 110 to 125 gr at 2,000 to 2,400 fps depending on barrel length, turn the same gun into a 200-yard carbine. Twist is what lets one barrel do both. At 1,050 fps a 220 gr bullet in a 1:8 barrel is only marginally stable, and the 240 gr and heavier monolithic solids are not; one turn in five inches spins every subsonic bullet made to full stability from 7.5″, and at the 2,000 to 2,100 fps a short barrel produces, jacketed supersonics are well inside their limits.
A 7.5″ barrel is the length the cartridge was built around. Subsonics lose almost nothing to a 16″; supersonics give up about 300 fps. The Faxon 7.5″ Gunner 1:5 in nitride (Match Series) has been Faxon’s best-selling barrel since 2025, and the MATCH-X version is that barrel with the bore honed and lapped and the exterior left bare.
Hornady factory data — 16″ test barrel
| Load | Muzzle | 100 yd | 200 yd |
| 190 gr Sub-X Subsonic | 1,050 fps / 465 ft-lb | 998 fps | 955 fps, −33.4″ |
| 110 gr V-MAX (BLACK) | 2,375 fps / 1,378 ft-lb | 2,094 fps | 1,834 fps, 0 (200 yd zero) |
Source: Hornady published ballistics from a 16″ test barrel. From the 7.5″ MATCH-X expect subsonics roughly 20 to 50 fps slower and supersonics roughly 300 fps slower. Hornady specifies a 900 fps expansion floor for Sub-X; a 7.5″ clears it with margin.
What You Need to Build It
Standard 5.56 bolt carrier group, upper, and barrel nut; 300 BLK-specific magazines are recommended for heavy subsonics. The 7.5″ takes a .750″ gas block and a pistol-length gas tube. The .104″ port is sized to cycle subsonics unsuppressed, which is the most gas the cartridge ever needs; add a suppressor or switch to supersonics and the adjustable block is what keeps it from over-gassing.
What Every MATCH-X Barrel Shares
416-R stainless bar stock, drilled, reamed to 32 Ra, honed to 8 Ra before the rifling button goes through, 5R button rifled, stress relieved at 1150°F, contoured, chambered, then lapped back to a uniform 16 Ra from throat to crown. Bore and groove air-gauged at three points, honed finish read on a profilometer, borescoped, chamber verified on a CMM, magnetic-particle inspected, nickel-Teflon extension torqued to 150 ft-lb, pinned, and headspaced. Exterior left as bare stainless blasted with S110 media. Made in Greater Cincinnati, Ohio, backed by the Faxon Lifetime Guarantee. See the whole MATCH-X line and the process video ›
MATCH-X 300 Blackout FAQs
Why a 1:5 twist for 300 Blackout?
Because subsonic bullets are long, slow, and heavy, and stability comes from spin. A 1:5 twist fully stabilizes every subsonic bullet made, including 240 gr and heavier solids that a 1:8 cannot, and it still shoots 110 to 125 gr supersonics at the velocities a short barrel produces.
Do I need an adjustable gas block?
Yes, if you will run a suppressor or switch between subsonic and supersonic ammunition, which is nearly every 300 BLK build. The Faxon .750″ adjustable block meters the port to each combination with no screw in the gas path.
What do the three rings mean?
Three rings behind the muzzle threads is Faxon's mark for 300 Blackout, so the barrel is never mistaken for a 5.56. A 300 BLK round will chamber in a 5.56 barrel and destroy the rifle; mark your magazines and uppers the same way.
MATCH-X or the Match Series 7.5″?
Same barrel, different finish. The Match Series (nitride inside and out) is Faxon's best-selling barrel and the lower-maintenance choice. MATCH-X hones and laps the bore and leaves the exterior bare for the smoothest possible bore on a barrel that will live behind a suppressor.
