What does the 12 in 12-bore actually mean and what makes a 20-bore what it is? How does it happen that the larger the number, the smaller the bore? Here's our easy guide that explains all.

Explaining shotgun bore sizes

Years ago, before kilograms and grams , things were measured in pounds and ounces and that is how shotgun bores got their names.

  • The size designation of a shotgun bore is based on a unit of weight based in pounds (llbs). Every pound is divided into 16 ounces (oz).
  • The link between this and the shotgun is that bore size is based on a spherical lead ball that fits the bore and the number of these that it takes to make up a single pound (1lb) in weight.

The term “bore” refers to the empty interior space of a gun barrel through which the ammunition is pushed through when a gun fires.

For example, a 16-bore barrel will accept a lead ball that weighs 1oz, which is 16 to the pound. At the other end of the scale a 4-bore means four balls to the pound, each ball weighing 4oz or ¼lb. So a 28-bore being smaller is 28 to make up a pound, and so on. So you can now see why the larger number means a smaller bore size and vice versa.


Cannons were also once known by the weight of the ball they threw. However as they are larger this gave sizes such as four- and 12-pounders.

A 16-bore shotgun cartridge

A 16-bore shotgun cartridge

Comparative weights

  • With the smoothbore shotgun the weight of the spherical lead ball bears a fairly close relationship to the appropriate shot load.
  • Basically, that means a weight of shot (and suitable powder charge) that was both practical and comfortable to shoot relative to the bore size and weight of the gun.
  • Some of the old “standard” loads work out spot on, such as the now obsolete Eley Grand Prix 8-bore with its 2oz load, which is exactly eight to the pound.
  • The word “gauge” is sometimes used as a substitute for “bore” — a practice that is still common in the US. While the UK sportsman will almost always refer to his gun by bore size, his gunsmith is more likely to check the gauge.
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Modern day measuring

The bore comparator allows for greater flexibility in measuring. This is especially useful as the problem with shotgun bores is that even for a given nominal bore there are minor size variations. Modern production techniques such as hammer forging produce barrels to an exact size, but in the days of spill boring and lead lapping this was not possible.

With muzzle-loaders this was of no great consequence as you loaded to suit with wads cut with a punch that was a match for the bore. The advent of the breechloader, with its powder and shot load conveniently packaged in a cartridge, might have been thought to reduce this sort of flexibility. This was not a problem, though, and was accommodated quite simply by using a range of bore sizes for each chambering. This explains how one can come across such seeming anomalies as a 16-bore stamped 17 by the proof house.

  • Take the ubiquitous 12-bore, which has four bore sizes represented for a great many years by either whole numbers or vulgar fractions. So, it is quite possible to find older guns stamped either 13, 13/1, 12 or 12/1. If this seems a bit odd, there are nine listed bore sizes for the 8-bore, and 4-bore barrels start at a gauge size of six.

As a very general rule the bigger the bore, the more subdivisions; the smaller gauges, less so. The 28-bore, for example, has three size divisions — 27, 28 and 29.

With a double gun, even one in original condition, the two barrel sizes may not be stamped up the same. With one that has been worked on in the bores and reproofed they may be quite different and even carry proof marks from a later period marked to another system. A very early breechloader marked up 12B 13M means that the main bore is gauged at 12 and the muzzle is gauged at 13, so it is choked. Not for ball was also an old way of signifying a choked barrel.

Showing the Proof Mark

Showing the Proof Mark

Changing the rules of proof

Amazingly, this archaic, if charming but slightly confusing system of expressing the bore size in whole numbers and vulgar fractions stayed with us until 1954, when there was a major change to the rules of proof.

Subsequently, the proof house stamped on the bore size in thousandths of an inch.

  • The range of 12-bore barrel sizes was now marked up .710, .719, .729 and .740. This did not signify any dramatic change, as your favourite gun on reproof was not destined suddenly to become a “seven-two-nine”. Its designation would remain faithful to the old lead ball/weight related system. It is still the case now, even under metric rules of proof.
  • There is one oddball, the .410, which is a bore size measurement and not weight related. Just think of it instead as a 36-bore, as some makers still do.