Complete Guide to Shotgun Cartridge Sizes and Chamber Lengths
- Sep 7
- 2 min read
When buying or using a shotgun, you’ll notice markings on the barrel such as “12/70” or “12/76 MAGNUM.” These numbers indicate the bore size and the chamber length (the maximum cartridge length your gun can safely fire).
Understanding ammo sizes like 60mm, 65mm, 70mm, or 76mm is critical for both safety and performance. Using the wrong cartridge can damage your gun or even cause injury.
Common Shotgun Ammo Sizes
Shotgun cartridges are measured by length after firing (when the crimp is fully opened). That’s why a 70mm cartridge may measure slightly shorter when unfired.
Metric (mm) | Imperial (inch) | Typical Barrel Marking | Common Use |
60 mm | 2⅜″ | Rare, vintage guns | Obsolete European shotguns |
65 mm | 2½″ | 2½″ chamber | Older British/European doubles |
70 mm | 2¾″ | 2¾″ chamber | Most common modern shell |
76 mm | 3″ | 3″ chamber (Magnum) | Magnum hunting loads |
89 mm | 3½″ | 3½″ chamber (Super Magnum) | Specialized waterfowl/turkey loads |
How to Read Barrel Markings
On most shotgun barrels or receivers, you’ll see markings such as:
12/65 → 12 bore, chamber length 65 mm (2½″)
12/70 → 12 bore, chamber length 70 mm (2¾″)
12/76 MAGNUM → 12 bore, chamber length 76 mm (3″ magnum)
20/76 → 20 bore, chamber length 76 mm (3″ magnum)
Safety Rules for Ammo & Chamber Sizes
Shorter Shells in Longer Chambers → Safe
Example: You can fire a 65mm cartridge in a 70mm or 76mm chamber.
The shorter shell simply doesn’t fill the chamber fully, but it is safe.
Longer Shells in Shorter Chambers → Dangerous
Example: Never fire a 76mm (3″) cartridge in a 70mm (2¾″) chamber.
The crimp will not open properly, causing dangerous overpressure that can damage the gun or injure the shooter.
Why Chamber Length Matters
Performance: Longer shells (76mm/89mm) carry more pellets and powder, giving extra power and range.
Recoil: More power also means heavier recoil, which can be tiring.
Versatility: A 76mm (3″ magnum) chambered gun can shoot both 70mm and 76mm shells, making it more versatile.
Understanding shotgun ammo sizes and barrel markings is essential for safe and effective shooting.
A marking like 12/70 means your shotgun can fire 70mm (2¾″) or shorter cartridges safely.
A marking like 12/76 MAGNUM means your shotgun can fire both 70mm (standard) and 76mm (magnum) cartridges.
Always remember: Shorter shells can go into longer chambers, but longer shells must never go into shorter chambers.
Next time you look at your shotgun’s barrel markings, you’ll know exactly what ammo to use — and why it matters.