Rifle · SAAMI (not in current Z299)
.338-378 Weatherby Magnum
A .338 built on the .378 Weatherby Magnum parent case — the largest commercially produced .338-caliber cartridge.
- Caliber
- 0.338″ · 8.585 mm
- Introduced
- 1998
- Case
- Belted
- Parent
- .378 Weatherby Magnum
- Primary use
- hunting_large_game, hunting_dangerous_game, long_range_hunting
- Status
- Active
- Introduced
- 1998
- Type
- Rifle
- Origin
- United States
- Inventor
- Elmer Keith and Bob Thomson (Weatherby commercialized 1998)
- Manufacturer
- Weatherby
- Standard
- SAAMI (not in current Z299)
- Status
- Active
- Availability
- Specialty
- Case capacity
- 131 gr H₂O
- Twist (typical)
- 1:10
- Primer
- Large Rifle Magnum
- Case type
- Belted
- Max pressure
- 63,817 psi, CIP transducer
- Eff. range
- 600 yd
- Max range
- 5950 yd
- Recoil
- 40–55 ft·lb
Introduced by Weatherby
Ballistics
Published velocity/energy data isn't available for .338-378 Weatherby Magnum. Parent case: .378 Weatherby Magnum.
Performance
Dimensionscase measurements · scale drawing · size vs familiar rounds
Case dimensions — the scale drawing at the top of the page is built from these.
CIP TDCC · approximate| Measure | mm | in | ✓ |
|---|---|---|---|
| Bullet ⌀ | 8.59 | 0.338 | · |
| Neck ⌀ | 9.35 | 0.368 | ~ |
| Shoulder ⌀ | 14.25 | 0.561 | · |
| Shoulder angle | 48.9° | — | |
| Base ⌀ | 14.78 | 0.582 | · |
| Rim ⌀ | 14.71 | 0.579 | · |
| Belt ⌀ | 15.34 | 0.604 | ~ |
| Rim thickness | 1.60 | 0.063 | ~ |
| Neck length | 10.80 | 0.425 | ≈ |
| Case length | 73.99 | 2.913 | · |
| Overall length | 95.58 | 3.763 | ~ |
Size vs familiar rounds
Drawn at true 1:1 — every case is accurate relative to the others. Absolute size depends on your screen's pixel density, so measuring one against a real ruler may be off. .338-378 Weatherby Magnum is highlighted below.
Missing a dimension for .338-378 Weatherby Magnum? Some case measurements aren't in a published SAAMI/CIP drawing yet. Have a factory or reamer print? Send it → — verified hard-to-find data earns a free Lindcott sticker.
Factory loads
Being verified
Advertised FPS, energy, and BC are withheld until loads clear verification.
Suitable powders
3 reloading powders listed for .338-378 Weatherby Magnum, fastest to slowest. Cross-reference in the finder →
Reloading costestimate cost per round vs factory
Estimate your cost per round and how it compares to factory. Inputs are yours — nothing is stored.
Cost estimate only — not load data. Charge weight is your input; follow published manuals for safe charges.
Firearms chambered in .338-378 Weatherby Magnum
Lineage
Connected reference
History
Developed by Weatherby as part of their tradition of extreme high-velocity cartridges. The .338-378 was initially a wildcat embraced by hunters seeking maximum .338-caliber performance before Weatherby standardized it in 1998. It targets dangerous game hunters who want the penetration and sectional density of .338-caliber bullets with velocity approaching .378 Weatherby levels. Recoil is extreme — typically exceeding 80 ft-lbs in a standard rifle — requiring either a very heavy rifle or a muzzle brake.
FAQs
- What twist rate does .338-378 Weatherby Magnum use?
- .338-378 Weatherby Magnum typically uses a 1:10 twist rate.
- What bullet diameter is .338-378 Weatherby Magnum?
- .338-378 Weatherby Magnum uses a 0.338″ (8.585 mm) diameter bullet.
- Is .338-378 Weatherby Magnum still in production?
- .338-378 Weatherby Magnum is active; typical availability is specialty.
- What is .338-378 Weatherby Magnum used for?
- .338-378 Weatherby Magnum is primarily used for hunting_large_game, hunting_dangerous_game, long_range_hunting.
- What is .338-378 Weatherby Magnum based on?
- .338-378 Weatherby Magnum is derived from the .378 Weatherby Magnum case.
- What are the best powders for .338-378 Weatherby Magnum?
- Reloading powders commonly listed for .338-378 Weatherby Magnum include Norma MRP, Reloader 25, Reloader 33.
Sources & provenance
- Standard SAAMI — SAAMI cartridge & chamber drawing — .338-378 Weatherby Magnum
- Manufacturer Nosler — 338 Caliber 250gr Partition (50ct) · representative_gun, grain_ballistics archived
- Manufacturer Weatherby — 338-378 Weatherby Magnum — Weatherby, Inc. (cartridge/store page) · representative_gun, grain_ballistics archived
- Manufacturer Weatherby — Weatherby Ballistics Charts — 338-378 WBY MAG · grain_ballistics, representative_gun
- Manufacturer Weatherby — Weatherby Ballistics Specifications Chart (PDF) · grain_ballistics, representative_gun
- Manufacturer Barnes — Barnes Load Data — 338-378 Weatherby Magnum (rifle/338/338-378-WBY.pdf) · representative_gun, grain_ballistics archived
Data & sources. Specs compiled from the Lindcott Armory reference; availability counted from 21 live factory loads; the trajectory is modeled (point-mass), not measured. See methodology. Spotted an error? Report it →
