Bore / Caliber
Definition
Bore is the internal diameter of the barrel. Caliber is that diameter expressed in inches (.32,.38) or millimetres (9 mm, 7.62 mm). Shotgun gauge is the inverse measure (smaller gauge number means larger bore).
- Bore
- Internal diameter of the barrel
- Caliber
- Bore diameter expressed in inches or millimetres
- Common inch examples
- .32, .38, .45
- Common metric examples
- 9 mm, 7.62 mm
- Shotgun equivalent
- Gauge, an inverse measure of bore size
Common questions
Why does a smaller shotgun gauge number mean a larger bore?+
Gauge is defined by how many lead balls of that bore diameter add up to one pound, so a bore that takes fewer, larger balls to reach a pound is given a smaller gauge number. A 12-gauge bore is therefore larger than a 20-gauge bore, the opposite relationship to how caliber numbers scale.
Can caliber alone identify the specific firearm that fired a bullet?+
No, caliber only narrows the field to firearms chambered for that size, and many different models and manufacturers share a common caliber. Individualising a bullet to one specific firearm requires comparing tool marks such as rifling striations, not caliber measurement alone.
Related terms
- Ammunition
- The cartridge assembly fired by a firearm. Comprises case, primer, propellant and projectile. Defined under Section 2(1)(b) of the Arms Act 1959...
- Cartridge
- The complete round of ammunition: case plus primer plus propellant plus projectile, assembled as a single unit for breech loading.
- Firearm
- A barrelled weapon that launches a projectile by the rapid expansion of gas from a confined burning propellant. Includes handguns, rifles and...
- Primer
- The small explosive cup at the base of a cartridge that ignites the propellant. Conventional primers use lead styphnate, barium nitrate and...
- Projectile
- The object expelled from the barrel. Single bullet in rifled arms, multiple pellets or single slug in shotguns.
- Propellant
- The chemical charge that burns rapidly inside the case to generate the gas that drives the projectile down the barrel. Black powder...
- Rifling
- The helical groove pattern cut into a firearm barrel that imparts spin to the bullet, stabilising it in flight. Rifling lands and...