"WWII US gun calibers: inches versus millimeters" Topic

Lately, while going through a list of various weapons used by the US military in WWII, I started to wonder whether there was any logic as to why some of the guns had their caliber expressed in inches, while others were expressed in millimeters. Or was it a somewhat arbitrary choice in that time period?

The smaller caliber armaments tend to be expressed in inches (e.g. .30 rifles, .45 pistol/SMG, .50 M2 MG), and these are cartridges that were designed (in US?) well before WWII. Also, some new weapons that were designed in US during WWII use the inch designation, such as 4.2" chemical mortar and 2.35" inch rocket launcher aka bazooka (as well as its post-war descendant 3.5" rocket launcher aka super bazooka).

Weapons built on licenced continental European designs seem to have used the original manufacturers millimeter designation, such as 40 mm Bofors gun and 60/81 mm mortars. However, also some apparently US designed weapons used millimeter classification, such as 57 and 75 mm recoilless rifles (seeing limited action in 1945).

Interesting case is the British 6 pounder anti-tank gun design, which was not referred by the original designation but re-named as 57 mm in the US usage.

Another interesting case is the 3" anti-tank gun (based on pre-WWII design) versus the 76 mm tank gun (designed during the WWII), which are actually the same calibre and even use the same rounds (though in different cartirdges). Two US designed guns of identical caliber designated differently – was that attempt to avoid confusion? If so, similar concern to avoiding confusion does not seem to have been applied to help separating .30 cal carbine rounds from .30 rifle rounds, which were not compatible.

Somewhere in the post-war years the US military seems to have gone fully metric when it comes to caliber designations, but the WWII era practices seem rather confusing to me.

It was an effort to maintain sanity in a world going metric mad. It failed.

What about the Mars mission where NASA confused metric with "Imperial" measures for dimensions?

Crunch. Splat.

Naval guns, the big ones, were all in inches. AAA was a confusing mixture.

A very interesting topic to post. Thanks, it could have been crossposted far more actually.

Just a guess, but in 1917-1918 the US Army replaced its domestically designed field guns with French designs to take advantage of the ongoing mass production arrangements (in France and the US) for those systems, rather than waste time and effort retooling to produce American-designed systems. That's similar to the reason why the AEF was equipped with an SMLE chambered for .30-06: easiest way to keep supply flowing without a lot of changes.

The French systems would've been entirely metric: firing tables, tolerances, etc. It would make sense to me if that drove the conversion.

Current US Navy weaponry still follows a mix of metric and imperial units. On the weapon systems I work with we freely talk in American and SI units. It's a mix of tradition, convenience, and a bit of American irasciblility in its engineers. It must drive our adversaries crazy.

The 3" ATG gun was derived from the old 3" AA gun, and so kept the imperial units. Likewise the 8" howitzer which was a development of the WWI British ordnance. The 4.2" mortar was also developed from a British design. Weapons that were licensed or developed from French designs, like the Brandt mortars, retained metric designations. The US Army also began using metric for new ordnance, hence the 37mm ATG and the 90mm AA gun. The 57mm, despite being a British design, was designated in metric because Ordnance would not countenance the use of "pound".

The designation of the 76mm gun was to clearly distinguish the ammo; as you noted it's the same projectile but with a different cartridge. An inverse case would be the bazooka, which was designated as 2.36" so as not to cause confusion with the 60mm mortar.

It's been twenty years since I looked at a dash ten manual, but my recollection of the Army's field artillery is similar- for maintenance, we used the English measurements soldiers would know and recognize with a degree of intuition, but for calculating firing data, most of the measurements (linear distance, velocity, etc) were metric, except weights.

in 1917-1918 the US Army replaced its domestically designed field guns with French designs

Thinking more of the topic today, I realised that in the post-WWI era there seems to have been a significant French influence in the US army heavy armament. Tank force was equipped with licence-built copies of the Renault FT-17 tank, infantry received 60 and 81 mm Brandt mortars (and even 37 mm trench guns), and field artillery got built around 75, 105 and 155 mm calibers.

Granted, some of that equipment had dropped off by the time US got involved in WWII.

The US Army also began using metric for new ordnance, hence the 37mm ATG and the 90mm AA gun.

Was the 37 mm cartridge an original US design, or licenced munition (there were lots of 37 mm AA and AT guns being designed in Europe in the Interwar period)? And if it was US design, would it have been the first US caliber to be given metric designation, or would there have been something earlier?

An inverse case would be the bazooka, which was designated as 2.36" so as not to cause confusion with the 60mm mortar.

Confusing bazooka and mortar rounds could lead to some confusion, I guess. However, IIRC in the case of the later super bazooka there should not have been too much of room for confusion with other weapons?

For a simplified summary, would the historical development have looked something like this:

- Before WWI: domestic designs and Imperial measurements
- During WWI: foreign (French) designs with metric calibers are introduced
- Post-WWI: more foreign designs (licenced) with metric calibers are introduced, and domestic designs start shifting into metric standard (with occasional use of Imperial measurement instead)
- Sometime post-WWII: new domestic designs primarily in metric calibers (NATO seems to employ metric system for common calibers)

On the weapon systems I work with we freely talk in American and SI units. It's a mix of tradition, convenience, and a bit of American irasciblility in its engineers.

Somehow this whole discussion makes me think of many English language wargame rules, which happily use a mixture of Imperial measurements (typically for table sizes and movement/shooting distances) and metric measurements (typically for base dimensions and miniature heights).

Was the 37 mm cartridge an original US design, or licenced munition

M3 gun and munitions were US designs. The Pak 36 was studied but the M3 ended up being its own thing.

However, IIRC in the case of the later super bazooka there should not have been too much of room for confusion with other weapons?

Speculation on my part follows:

1) The M20 was developed during WW2 but didn't see service. Possible that it was decided to continue with imperial units as per the M1 & M9 ammunition as troops would associate inches with rockets and millimeters with mortar bombs.

2) Another possibility is again avoiding confusion with other munitions. There was no other weapon with a 3.5" caliber. I've seen it's ammunition described as both 89mm and 90mm and I suppose the latter might cause confusion.

Well, the US Army had the screwiest system for identifying weapons and vehicles that could be imagined. At one point they had an M3 scout car, an M3 halftrack, an M3 light tank, an M3 Medium tank, an M3 howitzer and several other M3 items in their inventory! :)

Because there are three ways of doing things: The right way, the wrong way, and the Army way.

The 'French Connection' does seem to account for the use of mm rather than inch for the bulk of the US field pieces (seems the first 37-mm gun in US service was the M1916, so that may have been why the later M3 was also called 37-mm).

There were a few British examples as well, the 7.92-mm for the Besa and 9-mm for the Sten and also the 20-mm and 40-mm AA guns.

Gary

The British practice was similar to the US one, the Besa being a Czech design, Oerlikon Swiss, and Bofors Swedish. The Sten, although a thoroughly British creation, was chambered for standard 9x19mm Parabellum.

US Navy ordnance is similarly interesting in how it followed RN practice through WWI, with large and medium guns being in inches and smaller ones in pounds. Then between the War they stopped the British practice with small guns but also didn't adopt metric. Hence, the "Chicago Piano" AA guns were 1.1" rather than 28mm.

I imagine parental bloodline must have played a role. The Swiss Oerlikon and Swedish Bofors adopted by the USN retained their metric bore identifications in US service.

B

What Oerlikon designs did the US adopt?

As for the British practices, they seem so bizarre that I want to start another thread on the subject, once I get my thoughts collected.

The 20mm AA gun for one
link

Also via Sweden was the Bofors 40mm AA gun
link

As for the British practices, they seem so bizarre that I want to start another thread on the subject, once I get my thoughts collected.

True words.

In the early period of the Mediterranean campaign, the RN mounted captured 20mm Breda auto-cannon on their DDs and smaller ships to make up for shortages of light weapons. From what I recall reading, these guns were so well regarded that ships returning to GB for refit/repair would dismount these weapons to be mounted aboard other ships remaining in the operational theater.

B

I suspect that for weapons originating in Continental Europe, the manufacturing patterns and specifications were probably all metric, so it would not surprise me greatly if the manufacturing and maintenance processes for Bofors and Oerlikon weapons were "metricized."

I suspect that for weapons originating in Continental Europe, the manufacturing patterns and specifications were probably all metric

All the blueprints, manufacturing drawings, etc, related to the Bofors (as received from Sweden and Great Britain) were completely re-drawn/revised in order to meet US manufacturing standards and methods. This was particularly true with respect to the extensive amount of hand-fitting and finishing found in European factory processes.

Happy Holidays to all.
Pray for Peace in 2023.

B

What Oerlikon designs did the US adopt?

The 20mm AA gun for one

My bad for forgetting about 20 mm usage in the US (my main interest with US being on the Army ground equipment).

It seems that the US Navy at the time had quite a plethora of AA guns in use, while the Army tried to standardise on a more limited range (.50 cal, 40 mm and 90 mm), though also needed to keep 37 mm on limited use as it was the heaviest that could practically be mounted on a halftrack for self-propelled use (40 mm Bofors on halftrack was trialled with, but not approved for production). Though while the Army did not field 20 mm gun on ground mount, the Marine corps seems to have done that on some scale.

And then there were the 20 mm guns mounted on various aircraft, too.

The USMC used the 20-mm in their Defense, later retitled AAA, Battalions. There was also a 20-mm gun on the 1942 T/Os of the Marine Infantry Regiment, a dual purpose AA/AT weapon, though this does not appear to have actually been issued; .50-cal MG or 37-mm AT were the substitute items for it.

Gary