History
It started as a 1959 velocity experiment: neck the belted .378 Weatherby case, descended from the .416 Rigby, down to .30 caliber and see how fast a .30 could go. The answer was very fast. Weatherby finally cataloged it in 1996, and it remains one of the quickest factory .30s ever built. Light bullets break 3,400 fps; even a 220 grain match bullet clears 3,000.
The price is a huge powder charge, heavy recoil, and barrel life measured in hundreds of rounds. What it gives you is reach. Loaded with the heaviest high-BC .30 bullets, it shoots flat and hits hard well past 1,000 yards, which is the whole point of bringing it to the line.
Lineage
Necking the .378 case down to .308 inch (7.82mm) leaves the belt and the full-diameter body intact, so the cartridge feeds from a magnum-length action on a large rifle magnum primer. Its peers are the other big .30 magnums: the .300 Remington Ultra Magnum, .300 Weatherby, .300 PRC, and .300 Norma Magnum.
Specifications
| Spec | Value |
|---|---|
| Case type | Belted, bottlenecked (~106° venturi shoulder) |
| Bullet diameter | 7.82 mm (.308 in) |
| Neck diameter | 8.56 mm (.337 in) |
| Shoulder diameter | 14.23 mm (.560 in) |
| Body diameter | 14.78 mm (.582 in) |
| Belt diameter | 15.33 mm (.604 in) |
| Rim diameter | 14.71 mm (.579 in) |
| Case length | 73.99 mm (2.913 in) |
| Overall length | 95.25 mm (3.750 in) |
| Parent case | .378 Weatherby Magnum |
| Primer size | Large rifle magnum |
| Belted | Yes (headspaces on the belt) |
| Rifling twist | 1 in 9 in (recommended; factory ~1:10) |
| Max pressure | SAAMI 65,000 psi |
| Recommended barrel | 32 in, 1:9 twist |
Dimensions are taken from the maker case print (mm/in). It is a belted magnum that headspaces on the belt, and the radiused Weatherby venturi shoulder is drawn as a straight cone of the same angle on the cutaway.
Barrel Design
The twist I'd recommend is 1:9, faster than the roughly 1:10 of factory hunting rifles. The 200 to 230 grain high-BC .30 bullets that make this cartridge worth its recoil need that rate to stay stable through the transonic range. Lighter hunting bullets are happy in the slower factory rate.
Few .30s drink as deeply, and the slow powders want the full length of a long barrel to finish burning. I set the baseline at 32 inches to collect that velocity, and the tradeoffs are heavy recoil, a large charge per shot, and barrel life counted in hundreds rather than thousands of accurate rounds. The tables below are computed at that 32 inch barrel from the Weatherby factory loads; since muzzle velocity tracks barrel length so closely, confirm your own on a chronograph.
Match Ammo Performance
Nosler Trophy Grade · 210 gr AccuBond LR $5.73/rd
| Range (yd) | Velocity (fps) | Elevation (mil) | Energy (ft-lb) | Windage (mil) |
|---|---|---|---|---|
| 0 | 3200 | -0.3 | 4774 | 0.3 |
| 100 | 3060 | 0.0 | 4367 | 0.1 |
| 200 | 2924 | -0.3 | 3985 | 0.3 |
| 300 | 2790 | -0.7 | 3628 | 0.4 |
| 400 | 2659 | -1.3 | 3296 | 0.6 |
| 500 | 2531 | -1.9 | 2988 | 0.7 |
| 600 | 2408 | -2.5 | 2703 | 0.9 |
| 700 | 2287 | -3.3 | 2439 | 1.0 |
| 800 | 2171 | -4.0 | 2197 | 1.2 |
| 900 | 2058 | -4.9 | 1974 | 1.4 |
| 1000 | 1948 | -5.8 | 1769 | 1.6 |
| 1100 | 1841 | -6.7 | 1580 | 1.8 |
| 1200 | 1736 | -7.8 | 1406 | 2.0 |
| 1300 | 1635 | -8.9 | 1246 | 2.3 |
| 1400 | 1535 | -10.2 | 1099 | 2.5 |
| 1500 | 1439 | -11.5 | 966 | 2.8 |
| 1600 | 1346 | -13.0 | 844 | 3.1 |
| 1700 | 1256 | -14.6 | 736 | 3.4 |
| 1800 | 1171 | -16.4 | 639 | 3.8 |
| 1900 | 1101 | -18.4 | 565 | 4.2 |
| 2000 | 1063 | -20.6 | 526 | 4.6 |
| 2100 | 1034 | -23.0 | 498 | 4.9 |
| 2200 | 1008 | -25.6 | 474 | 5.3 |
| 2300 | 985 | -28.4 | 453 | 5.7 |
| 2400 | 964 | -31.4 | 433 | 6.1 |
| 2500 | 943 | -34.6 | 415 | 6.4 |
| 2600 | 924 | -37.9 | 398 | 6.8 |
| 2700 | 905 | -41.5 | 382 | 7.2 |
| 2800 | 887 | -45.2 | 367 | 7.5 |
| 2900 | 869 | -49.1 | 352 | 7.9 |
| 3000 | 853 | -53.2 | 339 | 8.3 |
| 3100 | 836 | -57.5 | 326 | 8.7 |
| 3200 | 821 | -61.9 | 314 | 9.0 |
| 3300 | 805 | -66.6 | 302 | 9.4 |
| 3400 | 791 | -71.4 | 291 | 9.8 |
Muzzle velocity 3200 fps is estimated at 32 in from the 24 in factory figure of 3040 fps at about 20 fps per inch. Expect your own barrel to read a little differently. Velocity is color coded green supersonic, yellow transonic, red subsonic; treat transonic and subsonic rows as approximate.
Trajectory
FAQ
What barrel length and twist should I run?
A 1:9 twist for the heavy 200 to 230 grain high-BC bullets that justify the cartridge at long range, and a 32 inch barrel to burn its powder charge. Factory hunting rifles use a slower twist around 1:10 for lighter bullets, which is fine for that role.
Is the 30-378 Weatherby a SAAMI cartridge?
Yes. The .30-378 Weatherby Magnum is SAAMI standardized at 65,000 psi, with factory ammunition from Weatherby. The case dimensions and cutaway on this page are taken from the maker print.
How does it compare to the .300 PRC and .300 RUM?
It holds more case capacity and more raw velocity than either, especially with lighter bullets. The .300 PRC and .300 RUM reach most of the same long-range performance with less powder, less recoil, and longer barrel life, which is why they dominate practical long-range shooting while the .30-378 remains a velocity flagship.
What is it good for?
Maximum-velocity long-range hunting and flat-shooting target work where reach matters more than barrel life or recoil. It is not an efficient or high-volume cartridge; the modern .30 magnums do that job. It is for the shooter who wants the fastest .30 on the line.