300 Blackout vs 556: One of These Will Destroy Your Rifle

A 300 Blackout round will chamber in a 5.56 barrel and destroy the rifle. Separate magazines, separate range days. Everything else is preference.

Last updated: September 14, 2026 · Originally published August 29, 2026

a 300 Blackout round will chamber in a 5.56 barrel and destroy the rifle. It is the single most important fact in this comparison, it has been documented on high-speed film, and it is the reason the two cartridges need separate magazines and separate range days. Everything else here is a preference. That part is not.

300 Blackout vs 5.56, short answer: Under SAAMI, 300 AAC Blackout and .223 Remington carry the identical maximum average pressure — 55,000 psi. The 300 Blackout exists to do two things a 5.56 cannot: make useful power from a very short barrel, and run subsonic while still cycling the action. Outside those two jobs, 5.56 is cheaper, flatter and available everywhere.

Quick answers

The kaboom, precisely stated

SAAMI publishes a list of unsafe firearm and ammunition combinations. The relevant entry reads:

In Firearms Chambered For: 223 Remington — Do Not Use These Cartridges: 5.56mm Military / 222 Remington / 25-45 Sharps / 30 Carbine / 300 AAC Blackout

And their general warning, which deserves to be on a wall in every reloading room:

“Remember just because a round of ammunition can fit into a firearm’s chamber, barrel or action does not mean it is safe to use that ammunition in the firearm.”

One precision point almost every article gets wrong. SAAMI lists 300 Blackout under the .223 Remington chamber entry. The separate entry for “5.56x45mm NATO” lists only 25-45 Sharps — it does not list 300 Blackout at all. Writers who say “SAAMI lists 300 BLK as unsafe in 5.56 chambers” are over-reading the document. The hazard is identical in practice; the citation is not what they think it is.

A citation hazard you should know about. As of August 2026, SAAMI’s live page for this document returns a 404 and the title no longer appears in their Technical Information menu. We could not determine whether it was withdrawn, relocated or simply reorganised. The only retrievable copy is the Internet Archive capture of the 24 August 2020 revision, which is what we are citing. If you need this for a policy or training document, contact SAAMI directly.

What actually happens

LFD Research filmed it on high speed. The .30 calibre bullet gets stuck in the neck of the 5.56 chamber, creating artificial headspace; the round fires; and the bullet is swaged down to .22 calibre and forced through the bore as if through a squeeze bore, with the lead core extruding forward out of the open tip. Their result: destroyed upper and lower receivers, magazine, barrel extension, bolt carrier and bolt.

Documented damage from a separate incident at rifleshooter.com: upper receiver in four pieces, barrel extension cracked with a sheared indexing pin, barrel nut fractured lengthwise, bolt carrier split lengthwise, extractor bent and separated, case head blown off and left in the chamber, charging handle latch sheared, magwell flared. No injury in that instance.

The published accounts disagree on the exact mechanism — bullet-jam headspacing, extractor headspacing, or bullet setback during chambering. The industry has not published a single authoritative account. The outcome is not in dispute.

The hazard is one-directional

A 5.56 or .223 round cannot chamber in a 300 Blackout barrel. Working from the SAAMI drawings: the .223 case shoulder sits about 1.438–1.467 inches from the base, while the 300 Blackout chamber shoulder is at roughly 1.066–1.089 inches. The round stops about 0.37 inches early and the bolt will not close. This is our reading of the drawings rather than a quoted source — we found no primary source stating it outright — but the geometry is unambiguous.

Practically: dedicated magazines per calibre, labelled storage, and one calibre per range trip. Magpul sells a 300 Blackout PMAG with deliberately different ribbing and texture specifically to give you a tactile and visual difference in the dark.

The .223 and 5.56 chamber question is a separate and much less dangerous one — we cover it in .223 vs 5.56, where the pressure figures turn out not to be comparable either.

300 Blackout vs 5.56 pressure: identical under SAAMI

We downloaded ANSI/SAAMI Z299.4-2025 and read the tables.

Identical pressure. The 62,366 psi figure you will see attached to 5.56 comes from NATO EPVAT using a case-mouth transducer, or from CIP using a drilled case — different methods, and as Wikipedia’s own entry states, “chamber pressures obtained using different methods are not comparable.” Note also that SAAMI does not standardise 5.56x45mm NATO at all; their cartridge is .223 Remington. We grepped the full standard to confirm it.

And look at that last column. SAAMI’s standard test barrel is 24 inches, with a written exception putting 300 Blackout at 16. So every side-by-side velocity table derived from SAAMI data is comparing a 24-inch .223 barrel against a 16-inch 300 Blackout barrel. Almost nobody discloses it. Hornady’s catalogue does the same thing — their .223 figures carry a “Test Barrel (24″)” label and their 300 Blackout figures say 16.

Short barrels: the actual design brief

SAAMI’s own reference velocities, 300 Blackout from 16 inches: 110 gr at 2,400 fps class, 125 gr at 2,185, 150 gr at 1,900, and the subsonic loads at 190 gr / 1,040 and 220 gr / 1,020.

Converted to muzzle energy, the defensible version of the “hits harder from a short barrel” claim is this: 300 Blackout 125 grain from a 16-inch barrel makes roughly 1,325 ft-lb, essentially matching .223 62 grain from a 24-inch barrel at about 1,306 ft-lb — from a barrel eight inches shorter. That comes straight from SAAMI figures and it is the honest form of the argument.

The stronger version — that 300 Blackout from 9 inches beats M855 from 16 — does not survive checking. Taking the widely-copied 9-inch figure at face value, 125 gr at 2,030 fps is 1,144 ft-lb, against measured PMC M855 at 1,183 ft-lb from a 16.1-inch barrel. The 300 Blackout number is lower.

And that 9-inch figure should not be quoted at all. The velocity table reproduced across dozens of articles traces to Wikipedia, where it carries an explicit “[citation needed]” tag. We confirmed the tag is present in the live article. Every downstream article launders it into apparent fact. We could not find a rigorous published cut-down test for 300 Blackout at 9, 10.5 and 16 inches with named loads.

The 62-grain M855 load that dominates bulk 5.56 is covered separately in our guide to green tip ammo.

Subsonic: the thing 5.56 genuinely cannot do

That SAAMI’s standard contains subsonic 300 Blackout reference loads at all is itself notable — 190 gr at 1,040 fps and 220 gr at 1,020 fps.

Measured from a 10.5-inch barrel into calibrated gel:

Subsonic 5.56 is impractical for three reasons. It will not reliably cycle an AR — insufficient gas port pressure to unlock the bolt carrier, making it effectively single-shot. The heaviest common .224 bullets are 77–90 grains, so at 1,050 fps you are looking at roughly 188–220 ft-lb against 300 Blackout’s 508 — about 2.3× the energy for the .30 calibre. And stabilising a heavy .224 bullet at subsonic velocity is marginal even in a fast twist, though we found no primary source quantifying that last point, so treat it as reasoning rather than evidence.

Suppressed: the advantage is the ammunition, not the can

A suppressor cannot suppress a sonic boom. That is the whole thing. Supersonic 5.56 cracks downrange no matter what is on the muzzle; subsonic 300 Blackout does not, while still cycling the action.

Measured decibel data does exist for 300 Blackout loads — a Larson Davis Spartan 821 meter, microphone one metre left of the shooter, five rounds averaged, from a 5-inch PDW with a 7.62 can. Hornady BLACK 208 gr A-MAX was quietest at 132.48 dBZ; the nine-load average was 134.18 dBZ.

What that data cannot tell you is how much quieter than 5.56 it is, because the test published no unsuppressed baseline and no 5.56 comparison. It supports load-to-load ranking only. We are not going to give you a “X decibels quieter” figure, because nobody has published one at citable quality.

Popular Suppressors, our sister site, works through a complete suppressed .300 Blackout build on a Bergara B-14 — the same trade-off applied to one rifle rather than argued in the abstract.

300 BLK vs 556 parts: what swaps and what does not

Verified directly against the SAAMI cartridge drawings, these dimensions are exactly identical: rim diameter .378, extractor groove diameter .332, rim thickness .045, breech face .3804, and maximum cartridge overall length 2.260.

That is the primary-source proof that the bolt interchanges — which is the 300 Blackout’s core design win, and the reason it succeeded where 6.8 SPC did not.

What differs: case length (1.760 vs 1.368), bullet diameter (.224 vs .309), twist (1:12 vs 1:8 in the SAAMI reference barrels), and chamber neck (.255 vs .337).

The barrel is the only mandatory change. Bolt, carrier and lower all carry over. Magazines physically work — AAC’s design brief required it — though Magpul’s dedicated 300 Blackout PMAG cites “modified internal geometry to allow for the large variance in projectile weights and shapes.” Note that Magpul stops short of saying standard PMAGs will not work, and we are not going to put that claim in their mouth.

Cost, checked and dated

LuckyGunner, 23 August 2026, cheapest in-stock per round: 5.56 at 44–46¢ against 300 Blackout at 54¢ for the cheapest, 75¢ for typical bulk FMJ, and 90¢ for the cheapest subsonic.

That is roughly 1.2× at the floor, 1.7× for typical bulk, and 2× for subsonic — which contradicts the common “300 Blackout costs two to three times as much” claim at the plinking tier while supporting it for subsonic and match. Single retailer, single day; check it yourself before budgeting.

What we could not verify

• The current official status of SAAMI’s unsafe-combinations document. Page and PDF both 404 as of August 2026; not in current site menus. We cite the archive and say so.
• Any original AAC or Remington design document. Every account of the design goals traces to secondary retellings. The goals are consistently reported; the primary source is not public.
• A rigorous published barrel-length study for 300 Blackout with named loads at 9, 10.5 and 16 inches. The figures everyone reuses are uncited at the source.
• Any doctrinal effective-range figure for 300 Blackout. The Army publishes one for 5.56 platforms — FM 3-22.9 gives the M4 a 500 metre point-target maximum effective range — but note that is a hit probability standard, not an energy threshold. Comparison articles routinely redefine it as an energy number and then compare. There is no equivalent published figure for 300 Blackout at all.
• The “burns all its powder in nine inches” claim. No primary source, no test data.
• Whether the pistol-length gas system on 16-inch 300 Blackout barrels is specified anywhere — it appears to be convention, not spec.

The practical rule

Buy 300 Blackout if you are building a short suppressed rifle and you will actually shoot subsonic through it. That is the job it was designed for and nothing else in the AR-15 pattern does it as well. The bolt and lower carry over, so it is a barrel swap.

Buy 5.56 for everything else. It is cheaper, flatter, more available, and the platform is built around it.

And whichever you own, if you own both: separate magazines, labelled storage, one calibre per range trip. This is the one comparison on this site where the wrong answer breaks the rifle.

Sources

• Maximum average pressure, test barrel lengths, case and chamber dimensions, twist, subsonic reference velocities — ANSI/SAAMI Z299.4-2025, downloaded and read directly
• Unsafe combinations list — SAAMI, revised 24 Aug 2020, via Internet Archive
• High-speed film of the failure — LFD Research via The Firearm Blog
• Documented damage — rifleshooter.com
• Manufacturer warnings — Windham Weaponry, NRA, Magpul
• Subsonic gel testing — AmmoMan; suppressed sound measurement — TargetBarn
• Measured 5.56 velocity by barrel length — The Firearm Blog; effective range doctrine — FM 3-22.9

5.56 against 7.62×39 →

James Nicholas

NFA Firearms Manufacturer and Professional Gunsmith. The XDMAN has a talent for taking complex firearms subject matter and breaking it down into an easy-to-understand format that all experience levels can relate to. James is an 07/02 NFA Firearms Manufacturer, a Professional Gunsmith with over 20 years of experience, and a Firearms Writer, Photographer and Firearms Expert. Connect with him on Instagram, X, and Facebook as @therealxdman.

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