Bore Derivation

Dear Technoid,

How were shotgun gauges determined? Is 410 a shotgun?


Dear Jimmy,

Shotgun gauges are determined in the following way. A one pound piece of pure lead is made into a certain number of balls. Let’s say that pound is made into 12 equal balls. The diameter of each ball would be .729″ and a shotgun with a barrel that size would be called “12 gauge”.

For a 28 gauge shotgun, the pound is divided into 28 separate and equal pieces and each piece is made into a ball. The diameter of each ball is .550″. Since 28 of them come from a pound of lead, a shotgun with a bore diameter of .550″ is called 28 gauge.

Technically, the 410 shotgun is called a 410 “bore”, not gauge. It uses the standard American measuring system of just measuring the diameter of the barrel directly. 410 bore shotguns have bores of .410″ diameter. 410 is the only widely produced shotgun designated “bore”. All the rest are in “gauges”.

All of this made sense when shotguns were muzzle loaders and could literally come in any “gauge” you could think of. In today’s market there are standardized shells sizes in 4, 8,10, 12, 16, 20, 24, 28, 32 gauge and 410 bore, but bore size is no longer standardized as much. It is quite possible to buy a 12 gauge gun today with bores ranging from .720″ to .745″. The gauge of the gun is now determined by the shell it shoots, rather than by the diameter of it’s barrels.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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