Specification
The numbers that define it
Dimensions: C.I.P. TDCC datasheets for .223 Remington and 5.56×45. Dimensional facts are not copyrightable; the drawings are not reproduced here.
Factory loads
Published load data
Muzzle velocities are the manufacturer's published figures, measured in their test barrel. Your rifle will differ — a shorter barrel typically loses roughly 5–15 m/s per centimetre. Chronograph your own load before trusting a long-range solution.
What it is
The 5.56×45 NATO and the .223 Remington share a case and a bullet diameter of 5.69 mm (.224 in). The commercial .223 came first, in 1963, out of Remington’s work on the .222 Special for the US Army’s small-calibre rifle programme. NATO standardised the military round in 1980 — but around the Belgian SS109 projectile (US designation M855, 62 gr with a steel penetrator), not the original 55 gr M193.
The differences between the two are real but narrow: 5.56 NATO is loaded to a higher pressure and its chambers have a longer leade (throat). Firing 5.56 NATO in a rifle chambered strictly for .223 Remington raises pressure; the reverse is safe. Most modern commercial rifles are cut to a hybrid “.223 Wylde”-style chamber precisely to avoid the question.
The twist-rate story
No cartridge illustrates the relationship between bullet length and rifling twist as clearly as this one, and it is worth knowing because it explains why the load list above is not interchangeable.
- The original AR-15 and M16 used a 1:14″ twist, chosen for the 55 gr M193 at summer temperatures. It proved marginal in Arctic testing — cold, dense air lowers the stability factor — and was changed to 1:12″.
- When NATO adopted the 62 gr SS109 in 1980, the barrels went to 1:7″. That is much faster than the 62 gr bullet itself needs. The reason is the M856 tracer, which is far longer than the ball round because a tracer’s pyrotechnic column takes up space at the base. Twist follows length, so the barrel had to be specified for the longest projectile it would ever fire.
- Modern 1:7″ and 1:8″ barrels will stabilise 75–80 gr match bullets. A 1:12″ barrel will not — a 77 gr Sierra MatchKing fired from one keyholes.
So the single most important compatibility question for this cartridge is not “5.56 or .223” but “what twist is the barrel”. Check it before buying heavy match ammunition. See twist rate basics.
Ballistically: fast, then not
A 55 gr bullet at 1006 m/s leaves the muzzle at about Mach 2.96 — genuinely fast, and flat enough inside 300 m that holdover is almost a non-issue. What it does not have is mass. The sectional density of a 55 gr .224 bullet is 0.157, against 0.264 for a 175 gr .308, so it sheds velocity far faster.
Run the S&B M193 load in the calculator and watch what happens: it is still above Mach 1.2 at 600 m, but by 800 m it is transonic and by 900 m the trajectory has fallen off a cliff. The heavy match loads — 69, 75, 77 gr — start 150–180 m/s slower but hold on considerably longer, and they are what people actually shoot at 600 m and beyond. That is the sectional-density trade in its purest form.
Wind shows the same pattern more brutally. A light, fast bullet spends less time in flight but has less inertia against a crosswind; a heavier match bullet in the same cartridge typically drifts noticeably less at 500 m despite the lower muzzle velocity. Set a 4 m/s crosswind in the calculator and compare the 55 gr and 77 gr loads at 500 m.
Barrel length matters more here than usual
Because the cartridge relies on velocity rather than mass, it is unusually sensitive to barrel length. Note the test barrels in the load list above: 600 mm for the Sellier & Bellot data, 20 in (508 mm) for the Hornady LE load, and just 16 in (406 mm) for Federal’s 69 gr Gold Medal Match. Those are not comparable figures, and treating them as if they were is one of the most common ways to end up with dope that does not match reality.
A rough guide is 20–30 m/s per inch of barrel lost from a 20 in reference, but it is very load-dependent. If you shoot a short-barrelled rifle, chronograph it. A 60 m/s velocity error is roughly 40 cm of drop at 500 m — much larger than any error the drag model contributes.
Use in Finland
The .223 Remington is widely used in Finland for fox, raccoon dog and other small game, and it is a standard practical- and service-rifle chambering. Finnish hunting legislation sets minimum energy and bullet-weight requirements by quarry class, and the .223 sits at the boundary of several of them — which is exactly the sort of question the energy tab is meant to help you think about, not answer legally. The thresholds live in the metsästysasetus; read the current text.