Why were all the calibres the same?

[frank3]

The comon WWII cannon-and MG calibres were

20mm
.303
7.76
.50
12.7mm
13mm

My question, why did all sides use the same calibres? (.50 and 13mm is very close, like .303 and 7.76mm)

Why weren't there like 15mm, or 11mm? Or heck, even 19mm?
Had it got anything to do with the 'ideal calibre'?

[Mitchell]

I thaught that the germans did have a 15mm, I can't remember the name of it thogh.

[Sable]

MG151 - the 15mm variant was installed in early Bf 109F models.

[Jester]

Originally posted by frank3
My question, why did all sides use the same calibres?
Had it got anything to do with the 'ideal calibre'?

A lot of the smaller ammunition - US .30 cal, British .303, etc. were originally made for Infantry Rifles. When the machine guns were developed the tried to use a common ammo round so that ammuniton could be shared and easier to produce because there wouldn't be so many types.

I think some of the larger rounds like the .50 cal, 20mm and 12.7mm started life as single fire anti-tank rounds and just progressed to machine gun ammunition.

[Tony Williams]

It was mostly down to historical accident, or inertia if you like.

Rifles and MGs of around .30 calibre (7.62, 7.7, 7.92 mm) became very popular world-wide at the end of the 19th century, partly no doubt because countries copied from each other but also because these were just about the most powerful cartridges which most people could comfortably fire from a rifle. Specific cartridges were spread through sales of the guns (e.g. the Italians and Japanese both used MGs in the British .303 calibre: they renamed it 7.7 mm)

The first HMG was the Geman 13 mm TuF of 1918, so that calibre (or something like it - the US and UK changed it to 12.7 mm as that measures 0.50 inch) became standard for HMGs.

The first 20mm cannon was the Becker of WW1, this was developed into the Oerlikon series interwar which became popular, so other manufacturers copied the calibre. It was also about the smallest to take a worthwhile quantity of HE.

The 37mm calibre was the result of an international agreement (the St Petersburg Declaration of 1868) banning HE or incendiary rounds weighing less than 400g-which was the weight of a 37mm shell at the time. Once you have the manufacturing capability set up for a partiocular calibre, it's easier to carry on making it, so you still get 37mm guns being made today.

Tony Williams: Military gun and ammunition website and discussion forum

[frank3]

I see I see, so basically, the .303 calibre was calculated to be the best calibre for a soldier to be comfortably fired!

It still is a strange calibre though...

[Jester]

.308 cal. - 7.62mm NATO is better IMO.  

Just better balistic and stopping power all the way around.  

[SMIDSY]

i prefer the old .303 round. why? they had a nasty tendency to tumble uppon hitting flesh.

[Squire]

The Russians had 23mm, the Germans had 30mm as well. Both were common.

Also, gun designers tended to copy ideas from one another (some simply making exact copies), and used similar rounds because of that.

Also, the rounds themsleves had been "tried and tested", and so trends in shell designs emerged, and good proven types were usually sought out. Militaries by their nature are very conservative organisations (I don't mean politically), and they like whats already been decided on as reliable technology.

[frank3]

So why weren't there more of the (for those days) uncomon types? What would be wrong with a 17mm calibre? It's lighter than a 20mm cannon, but has bigger rounds than the 13mm! (I have no idea if it even existed though)

[Oldman731]

Originally posted by SMIDSY
i prefer the old .303 round. why? they had a nasty tendency to tumble uppon hitting flesh.

All full-jacketed bullets have this tendency.

- oldman

[Squire]

17mm for instance, lets look at that. For starters, the round would have to be designed from scratch, and there is no gun that fires it. All the good, reliable 20mm, 15mm, 13.1mm and 12.7mm guns cannot chamber it.

So you need to develop, from scratch, a new gun, and round. The Air Ministry is going to ask "why spend all this time and money on this when we can chamber it for an exisiting 20mm round?". If your answer was "gee it sounds different", they will likely thank you for your proposal and shelve the idea.

Why wouldn't you just use a 20mm shell? There is no great advantage in using a 17mm weapon. Its large enough that the difference is too slight to have any great meaning. You would be unlikely to convince an air force to spend the $$$ and time just to have a 17mm weapon that may not work as you had hoped, and that may be no better than the 20mm one you have available already, that you know works.  

Also remember shell design is not just about the "mm" its about the weight of the round, in grams, and the powder it has, and the the HE capacity (if any).

In other words, you can have many different 12.7mm rounds, and they can all be different in power and range. They are not all made the same. Also, they can be tracer, HE, API, AP, ect ect. Same for the other rounds.

In the end, there has to be a *demonstrated need* <  to have the new round and gun, and most often, existing calibers will suffice. Its not a fashion show, where you wear a "new look" to be cool and trendy.

Its a good Q. though, and it is interesting to look into it, don't take my posts as slamming you for asking.  I'm not.

[Tony Williams]

Originally posted by Oldman731
All full-jacketed bullets have this tendency.

True, but the don't all tumble at the same rate. The .303 Mk VII ball (standard in both World Wars) had a light-alloy tip-filler which shifted the CG to the rear. That encouraged faster tumbling.

Tony Williams: Military gun and ammunition website and discussion forum

[Tony Williams]

Originally posted by Squire
In the end, there has to be a *demonstrated need* <  to have the new round and gun, and most often, existing calibers will suffice. Its not a fashion show, where you wear a "new look" to be cool and trendy.

I may be doing you a disservice here, but I have the impression you are mixing up two different things: the calibre in the strict sense of meaning the diameter of the projectile, and in the loose sense of meaning the cartridge a gun is chambered for.

If you take 20mm guns, for instance, there were many different cartridges used in this calibre, and none of them was interchangeable. So for armed forces which already had a 20mm gun it really didn't matter whether a new gun was in 20mm, 17mm or 23mm if the 20mm version was going to use a different cartridge from the old 20mm.

The only benefit from sticking to a particular calibre when designing a new cartridge was for the manufacturers as it meant they could use existing barrel-making and projectile-making machinery.

Tony Williams: Military gun and ammunition website and discussion forum