Autocannon · Not SAAMI standardized · Not standardizedObsolete
20x138mmB
20x138mmB is a belted bottlenecked autocannon cartridge.
- Caliber
- 0.7874″ · 20 mm
- Introduced
- 1930
- Case
- Belted
- Primary use
- Military
- Status
- Discontinued
- Obsolete
- Obsolete
- Introduced
- 1930
- Type
- Autocannon
- Origin
- Switzerland
- Inventor
- Swiss Solothurn (approx: maker of record)
- Manufacturer
- Swiss Solothurn
- Standard
- Not SAAMI standardized · Not standardized
- SAAMI
- Not SAAMI standardized
- Obsolete
- Obsolete
- Status
- Discontinued
- Availability
- Rare
- Case capacity
- 728.4 gr H₂O
- Twist (typical)
- 1:24
- Primer
- Large Rifle
- Case type
- Belted
- Max pressure
- 41,600 psi, CIP transducer
- Eff. range
- 1500 yd
- Max range
- 4500 yd
- Recoil
- 690.3–711.1 ft·lb
Ballistics
Velocity only, no drag model: no published ballistic coefficient is on file for 20x138mmB, so this is the shape gravity alone gives the advertised velocity. Drag would steepen it. The tool below solves real loads through their published G1 or G7 model.
Open in Ballistic Compare- Parent case
- —
Representative trajectory — modeled from a single velocity input (no published BC for this cartridge yet).
Dimensionscase measurements · scale drawing · size vs familiar rounds
Case dimensions — the scale drawing at the top of the page is built from these.
Wikipedia · approximate| Measure | mm | in | ✓ |
|---|---|---|---|
| Bullet ⌀ | 20.00 | 0.787 | · |
| Neck ⌀ | 20.90 | 0.823 | · |
| Shoulder ⌀ | 25.50 | 1.004 | · |
| Shoulder angle | 25° | — | |
| Base ⌀ | 26.86 | 1.057 | · |
| Rim ⌀ | 28.48 | 1.121 | · |
| Belt ⌀ | 27.86 | 1.097 | |
| Rim thickness | 2.90 | 0.114 | |
| Neck length | 20.66 | 0.813 | |
| Case length | 138.00 | 5.433 | · |
| Overall length | 202.70 | 7.980 | · |
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. 20x138mmB is highlighted below.
Missing a dimension for 20x138mmB? 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.
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.
Connected reference
History
Introduced around 1930.
FAQs
- What twist rate does 20x138mmB use?
- 20x138mmB typically uses a 1:24 twist rate.
- What bullet diameter is 20x138mmB?
- 20x138mmB uses a 0.7874″ (20 mm) diameter bullet.
- Is 20x138mmB still in production?
- 20x138mmB is discontinued; typical availability is rare.
- What is 20x138mmB used for?
- 20x138mmB is primarily used for military.
Sources & provenance
Data & sources. Specs compiled from the Lindcott Armory reference; the trajectory is modeled (point-mass), not measured. See methodology. Spotted an error? Report it →
