7.62×51 NATO / .308 Winchester

What it is

The 7.62×51 NATO and the .308 Winchester are, for practical purposes, the same cartridge: a 51 mm rimless bottleneck case firing a 7.82 mm (.308 in) bullet. They came out of the same US Army programme in the early 1950s — the experimental T65 case, shortened from the .30-06 to suit the newly practical ball-powder propellants — and Winchester released the commercial version in 1954, a few months before NATO adopted the military one.

The two are not identical. NATO specifies its pressure at a different measuring point and to a slightly lower limit than C.I.P. does for the commercial round, and military brass is typically thicker-walled with less internal capacity. In practice commercial .308 chambers safely and shoots accurately in 7.62 NATO rifles and vice versa, but a rifle throated for one is not necessarily throated optimally for the other. This is a well-worn argument; the aerodynamics this site models are unaffected by it.

Why it matters here

The .308 is the best single cartridge for demonstrating what this calculator is for, because it spans the entire interesting range of external ballistics in one case:

• A 147 gr ball load at 850 m/s is fast, light and gives up velocity quickly.
• A 175 gr match load at 790 m/s starts slower and passes the 147 gr in retained velocity somewhere around 500 m — the sectional density argument made visible.
• A 200 gr subsonic load at 320 m/s never breaks the sound barrier at all, and shows what a trajectory looks like when the bullet carries a sixteenth of the energy.

Load all three into the calculator in turn and the shape of the problem becomes obvious in a way no amount of prose achieves.

The 1000-yard problem

The .308 is the cartridge that taught long-range shooters about the transonic band. A 175 gr match bullet leaving at 790 m/s slows to Mach 1.2 somewhere around 800 m in standard air, and crosses Mach 1 near 880 m. A 1000-yard (914 m) match therefore has the bullet arriving inside the transonic band, where its own dispersion grows and small variations in muzzle velocity produce large variations in drop.

This is not a defect of the cartridge — it is the direct consequence of a 7.82 mm bullet at 790 m/s — and it is precisely why 6.5 mm cartridges (6.5×55, 6.5 Creedmoor, .260 Rem) displaced the .308 in long-range competition. Their higher-BC bullets stay supersonic past 1000 m from similar velocities.

Change the temperature in the calculator’s atmosphere panel and watch the transonic distance move. At −20 °C the speed of sound is 319 m/s instead of 340 m/s, so the bullet is “still supersonic” considerably further out. That effect is real, and it is why cold-weather dope differs from summer dope by more than air density alone explains.

Use in Finland

The .308 Winchester is one of the most common hunting calibres in Finland, used for everything from roe deer to moose, and it is the standard chambering for the reserve rifle-shooting disciplines. The Finnish twist convention on hunting rifles is usually 1:11″, which handles the 150–185 gr range that covers most local use.

Energy figures matter here for a legal reason: Finnish hunting legislation defines minimum energy classes at 100 m (E₁₀₀) for different quarry. The energy tab in the calculator prints E₁₀₀ for any load you enter. Treat that number as a study aid — the legal thresholds are set in the metsästysasetus and are amended from time to time; check the current text rather than a website.

Subsonic loads

Dedicated subsonic .308 loads sit at 310–325 m/s with a 200–220 gr bullet. Two things follow immediately.

First, they need a fast twist. A 200 gr .30 calibre bullet is long, and it is launched at 40 % of the usual velocity, so it receives 40 % of the usual spin. Lapua specifies 1:9″ for their 200 gr subsonic; standard 1:12″ .308 barrels frequently will not stabilise it.

Second, the trajectory is unrecognisable. The Sellier & Bellot 200 gr subsonic drops 6.7 metres at 400 m from a 100 m zero. Past about 200 m the dope changes faster than any practical scope can be dialled, which is why subsonic shooting is a short-range discipline regardless of what the rifle is capable of.

They also sit close to the barrier by design. At 325 m/s in +25 °C air (sound speed 347 m/s) a load is at Mach 0.94; at −25 °C (sound speed 315 m/s) the same load is at Mach 1.03 — supersonic. Cold-weather subsonic shooting is a real problem, and the margin manufacturers build in is exactly this.