How Much Does Barrel Length Affect .300 Blackout Bullet Velocity?

Test setup and .300 Blackout barrels

The video is filmed at Take Aim and Training Range in South Carolina and focuses on how barrel length affects .300 Blackout velocity. Ballistic Advantage supplied matching uppers so the comparison stays as close to apples-to-apples as possible. Barrel lengths tested are 5.5, 8, 9, 10, 12, and 16 inches, all with 1:7 twist rates and no muzzle devices to avoid adding velocity variables. A Garmin Xero chronograph records muzzle velocity. The rifle uses a Griffin SPR lower and an Aimpoint T2 on a Scalarworks mount, with no emphasis on accuracy or targets, only speed and feel. The uppers use Superlative Arms adjustable gas blocks and are effectively run like a bolt gun to remove gas system and suppressor back-pressure variables so the data reflects just barrel and ammunition behavior.

Subsonic and 110 gr supersonic velocity results

The test uses several .300 Blackout loads: 200 gr subsonic Magtech (referred to as Mac Tac), 110 gr supersonic Black Arc V-Max projectile range ammo, 125 gr Winchester brass-cased supersonic, and 145 gr Wolf steel-cased supersonic. The 5.5 inch barrel is shot first with subsonic and then 110 gr supersonic. The 110 gr load shows large velocity swings, with spreads of 100–200 fps between shots and an average around 1513 fps from the shortest barrel. The shooter notes that these 110 gr supersonic rounds feel noticeably “spicy” but inconsistent. This early data highlights that load quality and consistency can matter as much as barrel length when looking at velocity numbers and shot-to-shot variation.

Finding full powder burn around 9–10 inches

Moving to the 8 and 9 inch barrels, the 200 gr subsonic Magtech load becomes very consistent. In the 8 inch barrel, subsonic velocities cluster tightly around 1000 fps. In the 9 inch barrel, subsonic readings like 1063, 1038, 1067, and 1027 fps show similarly tight spreads. The shooter notes that around 9 inches is generally where .300 Blackout achieves full powder burn with subsonic ammunition, which matches the observed consistency. Supersonic 110 gr loads in these lengths still show noticeable variation, though the 9 inch barrel’s 110 gr data is somewhat better than shorter barrels, averaging about 1872 fps. This section establishes that 9 inches is a key length for stable subsonic performance and improved gas for cycling.

10 inch barrel with subsonic and 110 gr supersonic

With the 10 inch barrel, the 200 gr Magtech subsonic load remains extremely consistent, averaging about 1069 fps and staying well within the subsonic range. The rifle begins to cycle more reliably as barrel length increases and more gas is available. Switching to the 110 gr supersonic load, the chronograph shows velocities such as 1869, 1985, and 1770 fps, again revealing wide spreads and inconsistent performance from this particular ammunition. The average for the 10 inch barrel with 110 gr supersonic is about 1895 fps. The data reinforces that barrel length helps raise velocity, but ammunition choice and loading quality strongly influence shot-to-shot consistency, especially with lighter supersonic bullets in .300 Blackout.

12 and 16 inch barrels with subsonic and 110 gr loads

The 12 inch barrel pushes 200 gr Magtech subsonic rounds to an average of about 1091 fps, approaching the typical subsonic/supersonic boundary around 1130–1150 fps. Cycling improves and becomes more reliable. With 110 gr supersonic, the 12 inch barrel produces velocities around 1982–2089 fps, averaging roughly 2038 fps. Moving to the 16 inch barrel, the subsonic 200 gr load begins to cross into supersonic territory. Recorded velocities include 1122, 1138, 1158, 1145, and 1174 fps, with audible cracks on the higher shots. The shooter notes that a 16 inch .300 Blackout with 200 gr subsonic ammunition can easily push some rounds into supersonic speeds. The 16 inch barrel with 110 gr supersonic averages just under 2100 fps, around 2092 fps, which is less of a jump than expected over the 12 inch barrel.

Winchester 125 gr and Wolf 145 gr in short barrels

The test then shifts to two additional supersonic loads: Winchester 125 gr brass-cased and Wolf 145 gr steel-cased .300 Blackout. Starting with the 5.5 inch barrel, Winchester 125 gr produces velocities like 1751, 1724, and 1708 fps, showing very tight spreads and noticeably higher recoil compared to the 110 gr load. Wolf 145 gr from the same barrel gives readings such as 1582 and 1568 fps, slightly slower due to the heavier bullet but still consistent. In the 8 inch barrel, Winchester 125 gr jumps to around 1938–1927 fps, while Wolf 145 gr moves to about 1770–1710 fps. Both loads gain roughly 200 fps going from 5.5 to 8 inches, and both remain impressively consistent, especially compared to the earlier 110 gr ammunition.

Velocity gains in 9–12 inch barrels

With the 9 inch barrel, Winchester 125 gr supersonic shows velocities around 1975–1979 fps, with only about 20 fps difference between the lowest and highest shots. Wolf 145 gr steel-cased ammunition records about 1786–1777 fps, again with roughly 20 fps spread. The 10 inch barrel continues this pattern: Winchester 125 gr hits about 2020 fps, while Wolf 145 gr runs around 1821–1841 fps, with spreads in the 17–20 fps range. Both loads gain roughly 100 fps when moving from 9 to 10 inches. At 12 inches, Winchester 125 gr reaches about 2089 fps, roughly 70 fps more than at 10 inches. Wolf 145 gr at 12 inches sits around 1865–1898 fps, with about 35 fps spread. These results show diminishing but still meaningful velocity gains as barrel length increases through the mid-range lengths.

16 inch performance and velocity takeaways

In the 16 inch barrel, Winchester 125 gr .300 Blackout averages close to 2200 fps, with readings like 2196 and 2186 fps. This is only about 100–120 fps faster than the 12 inch barrel, less than the shooter expected. Wolf 145 gr steel-cased ammunition in the 16 inch barrel averages around 1970 fps, with a maximum near 1992 fps and consistent shot-to-shot performance. Comparing loads, the lighter 125 gr Winchester gains roughly 200 fps over the heavier 145 gr Wolf in the same barrel length. The shooter emphasizes that velocity is critical for energy and terminal effect, and that if running supersonic .300 Blackout, a consistent load like the Winchester 125 gr at around 2200 fps from a 16 inch barrel is appealing. The video closes by noting that all barrel lengths from 5.5 to 16 inches were tested with 110, 125, 145, and 200 gr ammunition to provide a fair, data-driven comparison.