.300 Blackout ammo buying guide

.300 Blackout is a .30 caliber AR-15 cartridge with a split personality: a supersonic hunting and defensive load that behaves like a short-range .308, and a heavy subsonic load designed to disappear behind a suppressor. They share a case, a magazine and a bolt, and they do not share much else. Buying the wrong one for your barrel length is the most common way to be disappointed by this cartridge.

Supersonic and subsonic are different jobs

Supersonic loads are the ones that behave like a short-range .308: a 125 grain bullet at about 2,200 fps from a 16" barrel carries useful energy to 200 yards, expands well on deer-sized game, and is the right choice if you never intend to suppress the rifle. They crack like a rifle round; you still need hearing protection.

Subsonic loads trade all of that reach for silence: a 220 grain bullet at about 1,020 fps, just under the speed of sound, so there is no supersonic crack when you fire it suppressed. The bullet arrives heavy and slow — closer in energy to a .45 ACP than to a rifle round — and the trajectory is steep enough that you should expect to hold over inside 100 yards. Unsuppressed they are an expensive and inaccurate way to shoot a rifle.

Barrel length and twist: the two numbers that matter

Barrel length is the first filter. A 16" barrel is the supersonic sweet spot and what most people build; a 9" barrel is optimised for subsonic work and still handles supersonic loads acceptably, if with a velocity cost. A 20" barrel buys a little supersonic velocity that most shooters will not notice.

Twist rate is the second. The heavy 200–220 grain subsonic bullets need a fast twist to stabilise — 1:7 is the standard recommendation, with 1:8 working for many loads. A slow twist barrel built for supersonic hunting loads may not stabilise the heavy subsonic bullets at all, which shows up as keyholing and absurd accuracy. Match the twist to the load you will actually shoot.

The cheapest in-stock price on the .300 Blackout page is currently $1.38 a round, which is a sensible floor for a practice supersonic FMJ. Subsonic ammunition, especially match-grade subsonic, costs more — and it should: those are precision-loaded heavy bullets, not bulk practice.

What to practise with, what to hunt with

For practice, supersonic FMJ is the volume option, and it is what almost everyone shoots in quantity. If you own a suppressor and care about subsonic performance, buy a box of the heavy loads and confirm your zero and cycling before you commit to a case; not every rifle cycles subsonic reliably without an adjustable gas block or a suppressor adding back-pressure.

For hunting and defence, a 110–125 grain expanding supersonic bullet is the right answer out to the ranges this cartridge honestly works at — call it 200 yards with a 16" barrel, less with a shorter one. If you are hunting suppressed and subsonic, choose a premium expanding subsonic load and accept the steeper trajectory: this is a deliberate trade, not a compromise to apologise for.

The gas system is the variable people forget when they change load class. A rifle tuned for supersonic ammunition may be over-gassed on subsonic — or, more often, under-gassed without a suppressor adding back-pressure. An adjustable gas block solves both problems for the cost of an afternoon with an allen key, and it is the single most useful upgrade if you intend to shoot both load families out of one upper. Cycling reliability is a gas problem more often than it is an ammunition problem.

The platform trap, in one line each

• Same bolt, same magazine as 5.56 — which is why the cartridge is cheap to convert to, and why the chamber mix-up above is so easy to commit.
• Same lower — no new lower receiver, no new magazines; the conversion is a barrel and a gas system.
• Label everything — if you own both calibers, keep them in separate, marked bags and never loose in a pocket.

If you are working out which of the two cartridges to build around in the first place, the long-action options are covered in the 6.5 Creedmoor guide, and the classic .30 caliber long action in the .30-06 guide.

A sensible first order

1. 100 rounds of supersonic FMJ to confirm cycling and establish a working zero for the barrel length you own.
2. Two boxes of your chosen supersonic hunting or defensive load for verification and carry.
3. If suppressed: one box of 200–220 grain subsonic to test cycling and point of impact before buying more. If the rifle will not cycle it without a suppressor or gas adjustment, solve that before you invest in a case.

Sorted by supersonic and subsonic, with current pricing, the shelf is on the .300 Blackout caliber page. The case-versus-box economics, which apply here the same as anywhere, are in price per round.