300 Blackout Velocity by Barrel Length: What Each Inch From 7.5 to 16 Inches Is Worth

The figures below are modelled from a 16 in reference and stepped down using per-inch loss rates that steepen as the barrel shortens. Real barrels vary by 30 to 60 fps at a given length from chamber and bore dimensions alone, so treat any single number as a centre point rather than a promise.

• ▸Atmosphere: 59°F, 29.92 inHg, sea level. Speed of sound at that temperature is 1,116 fps.
• ▸Powder: commercial short-barrel-optimised propellants for the supersonic loads, subsonic-specific powders for the heavy loads.
• ▸Barrel: nitrided 4150, standard 300 BLK chamber, chronographed 10 ft downrange.
• ▸Energy: bullet weight in grains times velocity squared, divided by 450,437.
• ▸Practical subsonic ceiling taken as 1,125 fps rather than 1,116, because transonic buffeting starts before Mach 1 and ambient temperature moves the line.

Read the delta column on the right. It is the fps gained per inch of barrel at that point in the ladder, and it is the figure that tells you whether the next two inches are worth carrying.

Between 12.5 in and 16 in you buy 69 fps for three and a half inches of barrel. Between 7.5 in and 11 in you buy 122 fps for the same three and a half inches. The cartridge is essentially finished burning by the low teens, and the last few inches are working on residual pressure that is already dropping fast.

There is a conclusion in that. If you want maximum supersonic velocity, 16 in is right and there is no argument. If you want a compact rifle, the sensible stopping points are 10.5 in and 8.5 in — 10.5 keeps you within 5 percent of a 16 in barrel's velocity, and 8.5 gives up 8 percent while removing eight inches of overall length.

The interval nobody should pick is 12.5 to 14.5 in. You carry most of the length of a 16 in build and give up velocity for it, and you land in the awkward zone for gas system selection where carbine gas is a little long and pistol gas is a little short.

Heavy loads move so little across this range that barrel length is nearly irrelevant to their velocity. What barrel length does affect, decisively, is whether a marginal load crosses into transonic. Below, a conventional 220 gr load and a hot 190 gr load that leaves a 16 in barrel at 1,140 fps — which is Mach 1.02, and supersonic.

That is the most useful thing on this page for anyone running a can. A load sold as subsonic is only subsonic at the barrel length and temperature the maker tested it in, and neither is usually printed. If you have a 16 in barrel and a supersonic crack coming out of a subsonic box, the ammunition is not defective — it is a 14 in load in a 16 in gun.

Verify with a chronograph rather than by ear. A borderline load will crack on a hot afternoon and be silent the same evening, and you will chase that inconsistency for months if you assume the ammunition is the constant.

300 BLK runs a shorter gas system than 5.56 at the same barrel length, because the cartridge produces less port pressure and needs longer dwell to make up for it. Port diameters below are the ranges commonly cut by barrel makers, and the suppressed-subsonic column is larger because a subsonic load behind a can is the hardest cycling condition this cartridge has.

Mid-length gas on a 16 in 300 BLK barrel is a minority choice and it deserves defending. It softens the impulse noticeably and runs supersonic beautifully. It also has a much harder time cycling subsonic, which is why carbine gas remains the default. Pick mid-length only if the rifle will live on supersonic ammunition.

One hard rule regardless of length: leave at least 1.5 in of barrel past the gas port. Ports cut closer than that to the muzzle lose pressure before the system has finished its work, and no buffer change will rescue it.

The solver takes a published velocity, the barrel length it was measured from, and your barrel length, then returns the corrected figure using the per-inch rates in the first table. It also flags when the source barrel length is unstated, because that is a table you should not be using at all.

Rule of thumb if you are doing it in your head: for supersonic 300 BLK, roughly 20 fps per inch in the low teens and above, roughly 35 fps per inch below 10 in. For subsonic, roughly 10 fps per inch anywhere in the range. Those are close enough to make a purchasing decision on.

Where the correction stops working is under about 7 in, where unburnt powder becomes the dominant term and the loss rate goes non-linear in a way a simple per-inch model cannot capture. Below 7 in you need a chronograph, not a table.

Speed of sound at 59°F is 1,116 fps. At 90°F it is about 1,150. At 20°F it is about 1,077. That is a 73 fps spread across ordinary shooting weather, which is more than seven inches of barrel is worth to a subsonic load.

The consequence catches people out backwards from how they expect. A hot day raises the speed of sound, so a marginal load is more likely to stay subsonic in summer. Cold weather lowers the threshold and, on some propellants, raises chamber pressure at the same time. A load that is quiet in July can crack in January, from the same barrel, out of the same box.

There is a second-order effect worth knowing: powder temperature sensitivity runs 0.5 to 1.5 fps per °F for most subsonic propellants. Combine a 40°F drop in ambient with a 40 fps drop in the sound barrier and a load with 60 fps of margin at 59°F can be sitting on nothing at all.

• ▸Chronograph your subsonic load at the coldest temperature you will shoot it, not the most convenient one.
• ▸Aim for at least 60 fps below Mach 1 on your coldest expected day. That is roughly 1,015 fps at 20°F.
• ▸If you own both a 9 in and a 16 in upper, do not assume a load qualified in one will behave in the other.

Velocity is the smallest part of it. A 7.5 in barrel gives up 233 fps of supersonic velocity against 16 in — real, but a tenth of the total. What you give up in larger measure is muzzle blast margin, sight radius if you run irons, and gas system tolerance.

Blast is the honest cost. An unsuppressed 7.5 in 300 BLK is unpleasant to stand beside and worse to shoot from a covered position, because a large fraction of the propellant is still burning at the muzzle. Unsuppressed short barrels in this cartridge are a compromise most owners come to regret; suppressed short barrels are the configuration the cartridge was designed around, and they are excellent.

The gas tolerance point is less obvious. A pistol-length system on a 7.5 in barrel has 3.5 in of dwell after the port, which is very little. Small changes in ammunition, fouling or suppressor back-pressure move that system in and out of tune far more readily than the same change would on a 16 in carbine-gas barrel. Build short if you want short, but budget for an adjustable gas block as part of the build rather than as a later fix.