Shotgun gauge to bore diameter
Gauge is the number of lead balls of bore diameter that weigh one pound together, and the diameter follows from that.
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How to use it
- Enter gauge and lead density.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Ball mass = 453.59237 / gauge (g). Volume = mass / lead density. Diameter = cube root of (6 × volume / π).
Example
Inputs: Gauge = 12, Lead density = 11.34 g/cm³. Result: Bore diameter: 18.53 mm; Diameter in inches: 0.73 in; Lead ball mass: 37.8 g.
Limits and caveats
- This is the theoretical definition; real bore sizes are set by CIP or SAAMI and differ slightly.
- The .410 bore is measured in inches, not gauge.
Questions about this tool
What is the bore of a 12 gauge?
About 18.5 mm, or 0.729 in.
Why is 20 gauge smaller than 12 gauge?
Twenty balls of its bore weigh a pound, so each ball is lighter and smaller than a 12 gauge ball.
What lead density is used?
11.34 g/cm³ for pure lead; you can edit it since the definition depends on it.