"WWII British gun calibres and their units" Topic

Two years ago, I started a discussion on this forum about the WWII US conventions for defining gun calibers, which were a curious mixture of inches and millimetres. You can find this discussion at:

TMP link

Since then, I have been occasionally occupied by trying to understand the British conventions of the same era. Being a total Johnny Foreigner myself, I must admit that I find the conventions in question occasionally a bit puzzling. It seems that the practices were driven by a curious mixture of tradition, pragmatism and eccentric exceptions intended to avoid confusion (not all of them successful). Following lists my impressions and understanding on the subject, corrections by better educated persons are most welcome.

1.) Projectile by pound

Very traditional way of measuring artillery weapons is by weight of projectile, dating back all the way to the 16th century. Royal Artillery, having long traditions, was still employing the practice at the time of WWII, using it to indicate field artillery pieces (18 pdr (*) and 25 pdr), medium artillery pieces (60 pdr) as well as the very modern anti-tank (a tk) guns (2, 6 and 17 pdr). The anti-tank guns could also be mounted on tanks as main weapons.

Being British, the projectile weight was measured in pounds, and the weapons in question were no doubt designed to provide "a proper pounding" for those in the receiving end. On the other hand, vehicle carry capacities were conventionally expressed in cwt (**) for trucks and tons for lorries.

2.) Imperial inches

More widespread convention for measuring armament calibres is by the muzzle diameter, which in case of British designed weapons firing British designed munitions was naturally in Imperial inches (in).

This was the convention used for small arms munitions, of which the most well known is the .303 rifle cartridge, widely used in infantry firearms. Some US designed weapons and munitions, provided as lend-lease aid, were also designated in inches, such as the .45 pistol cartridge (most famously used in Thompson machine carbine). Also, infantry mortars (2, 3 (***) and 4.2 in) used this convention, as did the Northover projector (2.5 in).

Many types of artillery pieces also employed inch designation, including heavy AA guns (3, 3.7, 4.5 and 5.25 in), medium and heavy artillery (4.5, 5.5, 6, 7.2 and 8 in) except for the previously mentioned 60 pdr, pack howitzer (3.7 in), as well as railway and coastal artillery,

3.) Millimetric measurements

Occasionally pragmatism took place over national pride, and the British military adopted weapons of foreign origins. Sometimes these were re-designed to employ British munitions, such as the .303 Bren gun (based on Czech ZB 26 design), sometimes not such as the 7.92 mm Besa tank MG (based on Czech ZB 53 design). Many such designs came from other parts of Europe, where the metric convention was strongly established, and whenever such weapon or cartridge was adopted also the metric nomination was typically adopted as such (rather than being converted into Imperial units).

In some cases, also British weapons were designed around foreign cartridges, such as the Sten machine carbine which was built to use 9 mm Parabellum pistol cartridge. Less well known is the 15 mm machinegun found in some of the early war tanks.

Metric convention could be particularly encountered in light AA guns, which were licence-built in UK in 20 mm (Oerlikon, Polsten and Hispano-Suiza) and 40 mm (Bofors (****)) calibres. Besides European designs, metric designation was also met in some of the more modern US designs, such as the 75 mm tanks guns mounted on Sherman.

A note on convention: in metric convention, there should be a space left between a value (number) and unit (e.g. 9 mm, not 9mm), as is also done with Imperial units (e.g. 3 in, not 3in). For unknown reasons this seems to still cause confusion to many British citizens.

4.) Bunch of spigots

British military probably has the distinctive honour of being the largest user of spigot mortars in WWII. Unlike conventional mortars, where a round is dropped into a muzzle of fixed diameter, spigot mortar has a metal rod, into which the round (having a matching hole in the bottom) is mounted. Thus in principle, rounds of varying diameters could be fired from the same weapon, though the weapon could be designated by the diameter of the spigot itself (which would naturally be smaller than the diameter of the fired round).

Probably the most wide-spread British spigot mortar of WWII is PIAT (*****). Other designs include the Blacker Bombard (allocated to Home Guard) and the Petard mounted on Churchill AVRE tank.

Notes:
* 18 pdr artillery piece may have actually fired a projectile weighing 18.5 pounds.
** cwt = hundredweight = 112 pounds
*** 3 inch mortar actually had a barrel diameter of approximately 3.2 inches
**** Designating Bofors gun (which fired 2 pound projectile) as 40 mm conveniently avoided confusing it with 2 pdr anti-tank gun (which had 40 mm barrel diameter).
***** PIAT = Projector Infantry Anti-Tank

And then there were the times that the designers seem to have had trouble deciding which convention to follow.

Most notably, when a new tank gun was designed for the Comet tank, this was designed to use existing projectile (from 17 pdr gun) mounted to a new cartridge, fired through a 3 in (76.2 mm) diameter barrel. Supposedly in order to avoid confusion, this most British weapons was designated as being of 77 mm calibre.

Speaking of the tank armaments, I am still trying to figure out what is the difference between 3.7 in and 95 mm tank howitzers, both of which were employed in various Cruiser CS variant tanks.

Not an expert on this, but my impression has always been that the Ordnance Board had a longstanding practice of classifying weapons based on the length of the barrel in calibers: short-barreled weapons (mortars and howitzers) were classified on the basis of bore diameter, while long-barreled guns were classified on projectile weight. That was generally true for land practice for the eras I am familiar with (Napoleonic, late XIX Century, WWI), even after the Board was abolished.

To quote Bill Bryson "It's often said that the British do this sort of thing because they like to confuse foreigners. Personally I think the Brits don't give a about foreigners, they just like to confuse themselves…"

Interesting, I seem to recall, way back in the mists of time, being taught not to put a space between the value and the units. I think I prefer it like that as it ensures the two stay together and don't get separated by a line break.
Also, you skipped a step: a hundredweight is 8 stone, each stone being 14 pounds (each pound being 16 ounces – perish the thought of using the same number twice?). And of course it's 20 hundredweight to the ton.
I think Nine pound round is right about the Ordnance system, it's just that it keeps getting messed up by Johnny Foreigner who will insist on doing things differently. ;o)

The Wikipedia article on the 77mm gun is pretty good imo.
link

Not an expert on this, but my impression has always been that the Ordnance Board had a longstanding practice of classifying weapons based on the length of the barrel in calibers: short-barreled weapons (mortars and howitzers) were classified on the basis of bore diameter, while long-barreled guns were classified on projectile weight.

I think that your name already makes you more qualified in this subject than I will ever be.

As for the practice that you mention, it seems to have fallen out of use by WWII, otherwise the new 4.5 in gun, firing 55 pound shells, would have been known as 55 pdr gun, which would have handily set it apart from the older 4.5 in howitzer, firing 34.5 pound shells.

Also, you skipped a step: a hundredweight is 8 stone, each stone being 14 pounds (each pound being 16 ounces – perish the thought of using the same number twice?). And of course it's 20 hundredweight to the ton.

Thanks for filling in this detail, though now that you mention it I am left wondering if the British military officially used the stone for anything during the WWII?

Of course technically speaking one could claim that the 2 pdr a tk gun was technically "1/7 stoner gun", though I do not think that sounds equally impressive – though if I were a tanker I would also be ashamed to admit that I had needed to bail out because my tank had been put on fire by some puny 1/7 pounder gun.

(That said, would "Rolling fourteen pounders" be a good name for a rock band? Not that I am thinking of starting one.)

Ha! I chose that because of my affection for the Napoleonic 9 pounder- a very nice piece of ordnance, in "Empire" terms (the mix of 9 and 6 lb guns is probably one of the best all-around mixes in those rules, since the 6lbs move as light and shoot as mediums, while the 9s are medium-heavy: a useful compromise).

I once had an English girlfriend who was as obsessed about her weight as only a good-looking woman can be, and she was constantly on about stones. The obsession is universal, but the different terms and the need for mental math to understand her gave it a charming he ne sais quoi, compared to hearing it the way I usually do.

At least we had a standardised system – one pound was the same throughout England from Medieval times. France had different measures for their livre in each city/town/region.

You still come across Brits who swear that the Imperial system was simpler than the SI system even though it obviously isn't.

Measurement of naval artillery is even more complicated between about 1840 and 1870 as guns are developing so fast that each designer/manufacturer/country seems to decide on their own system. Choose from: weight of the gun (with or without mounting), weight of the round shot (or shell), weight of the cylindrical shot (or shell) if rifled, calibre (either barrel dia or shot dia) etc. etc.

Don't expect the military mind to be organised – wargamers are not.

You still come across Brits who swear that the Imperial system was simpler than the SI system even though it obviously isn't.

Not just Brits. We Yanks think the same too. Not to mention that the Metric system is a Commie plot by those satanic French (!) Revolutionary rabble.

The Imperial system wasn't simpler but it was on a more human scale.

"I am left wondering if the British military officially used the stone for anything during the WWII?"

I think you will find the weights of individual people being recorded in stones (and pounds), as the vast majority of us still use stones for weight.

Ive always been faintly puzzled by the US insistence on measuring weight in (hundreds) of pounds, it is rather like measuring road distances in inches.

Annoyingly I have to convert stones and pounds into just pounds to log my weight on Fitbit. The Americans win again…

Try Australia, where weight is kilograms and height is centimetres. Although most people use kilograms and feet/inches because trying to picture a 179cm person is hard, but if you say 5'10" its clear.

The argument for the English system is quintessential British empiricism: the basic units are derived from units the ordinary person can intuitively understand, and represent extrapolations from his viewpoint. The argument for the metric system is scientific and mathematical- and as an artilleryman, I was always glad to be able to remember that a mil of angular difference translated into a meter at one kilometer- imagine how much more complex that becomes with English units!

But I still think reflexively in feet, yards, miles and pounds, and when I read or hear a metric measurement, convert it mentally into the corresponding English measurement, FWIW.

All I know us that fifty years of pumping the metric system up our backsides in Canada has been admitted as a complete failure, when in 2019 even the government insisted that everyone stay six feet apart.

I agree with NPR's comment about the human scale of Imperial measurements. In metric countries, a standard school ruler is 30cm long or 1 foot. In Dutch an inch is called a duim, the word for a thumb which was the first legal definition of an inch (the average of the width of three men's thumbs).

The US Army mixed and matched, and least in the field artillery- so that all of the muzzle velocities and distances in the firing tables were metric (m/s and meters), while projectile weights were reckoned in English pounds. And just to confuse it a little more, they used "squares" to denote the degree to which a projectile of a given caliber varied from its standard weight (usually an ordinary HE round) – a 33ib 105mm round was considered "two square," while a 96lb 155mm HE round was "four square."

Don't even begin to know where that came from.

Just as a note, the caliber of the petard mortar mounted on the AVRE is wrong almost everywhere.
link

He measured a surviving AVRE, in addition to the wonderful archival work he's known for.

There was also a 3lb gun on one of their interwar tanks.

There was also a 3lb gun on one of their interwar tanks.

You are probably thinking of Vickers Medium Mark I, which had been phased out of use just before WWII. For details, check out e.g. the Wikipedia entry (notice the funky side MGs in the picture):

link

BTW. It is a 3 pdr gun, not 3 lb gun!