One of the wonderful things about the world of the sporting firearm is that quite often change, when it comes, proceeds at an almost evolutionary pace. Nowhere is this more obvious than how we describe the size of a shotgun. Consider this: what does the 12 in 12-bore actually mean? What makes a 20-bore what it is? And how does it happen that the larger the number, the smaller the bore?

To find the answer we have to step back a long way in time, well before the breechloader was developed. The size designation of a shotgun bore is based not on a direct measurement but relates to a unit of weight, the pound (lb). Now, this may seem rather odd as, for those too young to remember, it is not many years ago that you bought your apples, sugar, flour and suchlike by the pound. For smaller purchases the pound was divided into 16 ounces (oz), and each ounce was divided into smaller fractional units for luxury purchases such as sweets.

The link between this and the shotgun is that nominal bore size is based on a spherical lead ball that fits the bore and the number of these that it takes to make up 1lb weight.

For example, a 16-bore barrel would 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, it follows that a 28-bore being much smaller is 28 to make up a pound, and so on. It becomes easy to see why the larger number means a smaller bore and vice versa.

This was not a system only applicable to shotguns as smoothbore cannons were once known by the weight of the ball they threw. However, being on a somewhat larger scale, this was in multiples of pounds, giving sizes such as four- and 12-pounders. Even rifles were at one time based on the same system and that is why one can find delightful descriptions in auction house catalogues such as 36-bore muzzle-loading rifle. As for a 4-bore rifle, it really did fire a ¼lb ball, or alternatively a conical bullet weighing half as much again at 6oz.

Comparative weights

Interestingly, 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. Most are fairly close, one exception being the 12-bore, which is nearer to what we would now regard as a heavy load for a 2¾in (70mm) cartridge. This should not come as a surprise as many 12-bore shot loads for game guns have been reduced over the years. In blackpowder days it would not be that unusual to use 1¼oz (36g) of shot, but now 1oz (28g) loads are popular, or to put it another way, 16-bore loads for a 12-bore. Of course, the gunmaker was not actually going to be fiddling about with lead balls and weighing scales, and so for a great many years the bore was checked for size using cylindrical plug gauges. This is a method that undoubtedly gave rise to the expression “gauging the bore”. In turn, this explains why the word “gauge” is sometimes used as a substitute for “bore” — a practice that is still common in the US. It is hard to say which is technically correct. However, while the UK sportsman will almost always refer to his gun by bore size, his gunsmith is
more likely to check the gauge.

Modern-day measuring

Following on from the plug gauge came the bore comparator, which does a similar job but allows for greater flexibility in measuring and more precise measurement. 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. Not only that, but sophisticated measuring equipment such as the bore comparator was not available, so you were working in the dark.

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.

Changes to 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 and I am happy to report that 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.