Bore diameter to shotgun gauge
This tool reverses the gauge calculation and gives the gauge from a bore diameter.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter bore diameter 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 volume = π × (diameter in cm)³ / 6. Mass = volume × lead density. Gauge = 453.59237 / mass.
Example
Inputs: Bore diameter = 18.5 mm, Lead density = 11.34 g/cm³. Result: Exact gauge: 12.07; Nearest whole gauge: 12; Lead ball mass: 37.59 g.
Limits and caveats
- The result is theoretical; modern barrels vary slightly by maker and choke.
- Measure the inside diameter past the chamber and before the choke for a meaningful reading.
Questions about this tool
What gauge is an 18.5 mm bore?
About 12 gauge.
What gauge is a .410 bore?
At about 10.4 mm it works out to roughly 67 to 68 gauge by the definition.
Why is the gauge fractional?
A measured diameter rarely matches a whole gauge exactly, so the fraction shows how close it is.