Cartridge case metric descriptions, 20+mm

I am currently researching cannon cartridges, 20-57 mm, and am now canvassing opinions on how to describe them. The usual method is to list the calibre and case length, with modifying letters to identify rim types other than rimless. This is so convenient for identifying different cartridges of the same calibre that it is applied retrospectively to cartridges which have never had any official metric designation, e.g. the 1.1 inch USN round can be found listed as 28x199SR.

However, once you get stuck into experimental as well as service rounds, you will find more and more examples of different cartridges with the same calibre and case length. The best example I know is 20x110; the well-known ones are 20x110RB Oerlikon S, 20x110 Hispano HS 404, and 20x110 USN (Mk 100 series). There were also some experimentals: 20x110 Scotti; 20x110 Russian AP20; and 20x110R Russian TsKBSV-75.

I have seen some instances of collectors who have tried to assist identification by adding a third measurement: the rim diameter. So the 20x110 rounds would become:
20x110x24.8 = HS 404
20x110x29.5 = USN Mk 100
20x110x21.9RB = Oerlikon S
20x110x31.0 = AP20
20x110x31.0R = TsKBSV-75
20x110x24.8 = Scotti.

This should make it much easier for collectors to identify cartridges.

You will immediately notice a problem, in that the number designations for the HS 404 and Scotti remain the same (the Scotti case has a tapered shoulder like the Oerlikon’s), so in that case “Scotti” would still need to be tagged on, but such instances are rare.

Any thoughts on this? Would it be worth doing, or is it too complicated?

What about showing them in tabular form, with an additional column for ‘comment’ or ‘variation’ here you could list ‘tapered shoulder’ for the Scotti. Or, you could add a footnote, which might be more elegant, where you could (as you have done here) point out that there are two rounds that share the same case length and rim diameter but different case forms.

Tony, I fully understand your thought.

Not knowing what is best but for me it appears sufficient to have an identifier like “HS”, “USN”, “Oe” or any other.

Looking at history we will notice that back in the old days it was not even usual to give a case length.

And adding a third measurement will hardly identify a caliber for a reader as none of us (including experts) will know off the top of their heads what a 20x110x31 will be. Just causing confusion.

In my very personal view the good old “20x110 HS” is the best way to do. All will be able to read and understand without trying to find a decoding list for head diameters or even worse getting lost in typos.

Let alone that many people (non ammo folks) have a hard time to understand that a “20mm caliber” is not the head diameter of a case.
Using lengty designations will make people think they are looking at a matematical formula for astronomy or so.

As said, just my view.

Besides that, of course it is always good to have caliber lists which are giving case head diameters in a complementary column.

Tony- I like the concept, used on a limited basis such as the 20 x 110 illustrated above. This is a quick and easy way for people with limited experience to quickly identify, or at least narrow down the correct identification of a round.
I doubt is this is something that would get instant or widespread adoption as a new standard for terminology on all calibers, but on a site such as yours it would be extremely useful.

I am sorry to see myself again having a different opinion than Alex has.
But for some time and with rifle cartridges in mind, I have been thinking that adding the case diameter to the caliber x case length “metric” description would be very useful.
Many prefer “names” like .264 USA, 6.5 Creedmoor, .300 Blackout, xyz Short Magnum, xyz Super Short Magnum etc. when referring to cartridges. But it is often quite difficult in my experience to keep track. There are simply too many of them.

If there were, in addition to such names, a standardized way of including the base diameter in the metric designation, we could immediately see that 7.62x33x9.1 is in a different power class than 7.9x33x11.9 (.30 Carbine versus German assault rifle). I am not at all proposing this for everyday use. But it would in my view be very helpful for organizing cartridge comparisons and collections.
The days when the bulk of rimless rifle cartridges had practically the same case diameter (that of 7.9 x 57 or .30-06) and outliers like 6.5 Sweden or Carcano did not really create problems are over. We now have a great number of different dimensions in use.

Obviously, you Tony and I independently came to very similar conclusions in our different fields of interest and see a need for more information in the designation, at least for specialist, not everyday use.

By case diameter I mean the P1 dimension as defined by CIP, preferably rounded to the next 0.1 mm. That is above the extractor groove, rim or belt. A standard, to avoid things like the current existence of 7.65x54 and 7.65x53 for the same Mauser cartridge, would be most useful. But I dont see a body that could decide on it. CIP surely is too arrogant to even consider such a standard.

I would love to also see the table include a column for old/alternative names. I have several rounds but for some of them I don’t know what they are properly called. For instance, I have no idea what the case dimensions are for a 2-Pounder. If such a table had a column with “2-Pounder” in it, I could then see the dimensions in the other columns. Another measurement that would help in identifying necked cases would be to include the neck/shoulder angle. With that being used it would be much easier to try to identify cases. Just suggestions. I don’t know anywhere near enough to try to do it.

Tony and JPeelen have similar goals, but Tony working on the rim diameter and JPeelen the base (ahead of the rim) diameter.
With small arms ammunition it is easy to get an accurate base measurement. But for larger caliber cases few collectors have a tool to measure the base, but can easily measure across the rim for rim diameter. Therefore I prefer Tony’s approach for larger calibers (say .50 caliber/12.7mm and above). Of course, as long as whoever decides to add a third measurement to the case ID specifies if it is rim or base it will be useful.

Gentlemen, try to look at the practical part.

To clarify it a bit here a reverse approach, what do these figures tell you?:
Being over 40 years in ammo it tells me this: nothing!
The RB will let me guess on the Oerlikon and the Swedish 20x110 HS with “long neck” will have the same measurements as the normal HS and still will require a “verbal explanation”.
What will be the gain in such a list where it still will have to be clarified and all numerical only “un-clarified” ones (like above) will cause nothing else but a lengthy search what for example “24.8” will indicate - then I will measure a fired case and come to “24.9” instead and heureka, have a new caliber?

As said, having a complementing column in a list giving head diameters sure will be good. But this should be a valuable addition only.

And pay attention that the question here applies only to calibers 20mm and larger.
What is the point to have this only 20mm and up?
All calibers below will be designated as before (7.62mm, 9mm and 12.7mm for example will be impossible to be handled this way) and only with 20mm+ this will kick in? What for?
Also think of all those 37mm and 75mm, 105mm and other calibers (plenty of them). There it is absolutely not unusual to have same rim diameters (see Hawkinson’s lists). Means there all the “verbal clarifications” have to be used too. Not to mention experimental calibers where explanations are immense important. As always figures make bad names.
So the whole “3rd numerical” will only be one more number to confuse people as the “verbal clarification” still will have to be used.
A contradiction in itself.

And measuring case heads and rims might bring an array of measurements for just one cliber as for these factors:

  • measurements may vary and not always match the “inventor” specs, think of case lengths, there it is to be observed daily. Let alone that fired cases may have different measurements (lengths and diameters).
  • many people are harldy able to use mechanical calipers and will create a lot of new calibers.
    This will bring as a huge grey area or figures and “verbal clarifications” will always be used as the discussions and arguments on measurements will kick in the very first day.

Reinventing the wheel is not easy.

Again only my personal view.

I think the point for Tony is he is researching cannon cartridges, so that is likely why the designations are for 20mm upwards.

That I know well but it will drag all other calibers into it and as said in 37mm, 75mm and up there will be lots of confusion. Besides my other points on 20mm+ still being valid.
So one needs to keep the wider view on all.

I agree with EOD but I really don’t see the need to convert rounds that were manufactured and known in imperial measurments into metric, after all as Tonys example of the 1.1 inch USN round is exactly that and known as such…28x199SR, what’s that?

TonyL, maybe designating it simply as:
28x199SR 1.1" USN

Using inch designations would be the opposite of Tony’s proposal as there the case length is missing.
Let alone that except for the US the rest of the world has little use for inch measurements.
The US military has switched to metric long ago and that must have been for a very good reason.

After all there is basically a metric designation for any inch-caliber which was used after 1945.
Like .30, .50, 3", 5", 8" and so many others. Even “late” inch calibers like .338 have metric designations in several militaries.
Exceptions like 1.1" and maybe few others shall not justify to run a second caliber table in inch to just have 5 calibers in there.
And speaking of wich, what was the French designation for the 1.1" USN?

I’d say it is easier to keep one system straight (i.e. metric + identifyer = 20x110 HS) instead of having several systems (lists) in parallel like
1.1" USN
4th will be “calibers commonly called wrong designations to please ignorant people” like 20x138B Solothurn or 7.62x39 AK… (sorry to Tony Williams, he knows what I mean)
5th will be gender oriented then I guess to keep it PC… (sorry, had to draw this one)

I know I am writing way too much in this thread but I spent my first life with medium calibers…
I can see people reading this and shaking their heads…

Still only my personal views.

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To me this system has one major flaw to begin with - the bore/proj diameter and case length are very rarely specific/accurate (most often rounded up/down) while in this system the head measurement would be accurate to 1/10th or 1/20th of a mm?

Proj diameter +/- 1,5 mm
Case length +/- 1,5 mm
Head diameter is accurate to 1/20th mm?

Here’s one accurate to 1/20th mm: 20,0x139,05x28,45 mm…
Even better one for a certain small round: 5,65x39,3x9,95


Thank you for all of your comments, gentlemen. Having considered them I am minded to retain the structure of the ammunition table I used in Rapid Fire, of listing the usual metric designation first (with additional identifier if necessary) and in the next two columns, the rim diameter and body diameter (widest part of case, excluding any rim or belt), with any further information in a final comments column.

So, for example, the 20 mm Scotti appears as:
20 x 110 (Scotti); 24.8; 24.9; Italy

While the 1.1 inch appears in the table as:
28 x 199SR; 43.5; 41.4; USA; WW2 1.1" naval AA

(typical projectile weights, muzzle velocities and muzzle energies are also given)

One difference from the structure of Rapid Fire will be that in addition to providing data tables, I will be adding chapters including a paragraph or more on each cartridge (by calibre) and cannon (by nationality) in the book. Examples of the headings to each of these paragraphs are:

(Ammunition): 1.1 inch USN (28 x 199SR)
(Cannon): 1.1 inch Mark 1/1: 28 x 199SR ammunition

I am still left with a few decisions concerning the metric measurements. As a general rule, the calibres and case lengths are given as whole numbers, e.g. 20 x 124 for the Japanese Ho-1 and Ho-3, rather than 20 x 124.5. Rim and base diameters are normally given to one decimal place. However, some of the British 1 inch rounds are officially designated 25.4 mm (including on headstamps), while some of the experimental British 20 mm rounds are not given metric measurements; they were officially 0.8 inch which is 20.3 mm - so should I use 20 or 20.3mm?

Decisions, decisions!

Why not for the paragraphs use the whole number and in the index, reference the variations to that paragraph/cartridge?.
That way it would capture various naming and measuring ID.

I think designating it as 1.1" USN (28x199SR) as per Tony Williams ‘Rapid Fire’ would make more sense as the imperial designation is instantly recognisable to collectors and the like as that was what it was when in use and was designated as such by the country of origin, much the same as the 50 BMG, the world and it’s dog knows what a 50 BMG is so why change it anymore than trying to change 20x110HS into imperial?
I do think having the metric conversion (not designation as it was never designated as that by the country of origin) is a good idea as a after note because of the extra information,in this example case length and that it is semi rimmed.
Same as you,just my personal opinion and from a person whos country is officially metric but officially also uses imperial in our everyday lives


Tony Williams:
I fully agree on your format.
As for conversions of inch to metric or naming case lengths. For myself I am doing this way:

  • taking the official inch caliber and then convert it with a calculator to exact metric numbers. Here then for example the .80" becomes 20.32mm and as per mathematics and the rules for rounding (from digit “5” it rounds upwards) it becomes 20.3mm. With calibers larger 12.7mm I am rounding only to 1 digit after the point. Small arms ammo I am rounding to 2 digits after the point.
  • basically same goes for case lengths, means 124.5mm becomes 125mm.

The most famous and often discussed is the Russian 4.2-line (.42").
Here it is .42" which is in metric 10.668mm, this then rounds to 10.67mm. For my self a clear issue.
But to my surprise there is plenty of different designations in metric, some from incorrect rounding and some from people who measured the true bullet diameter and ran with that.

So basically we both agree on the same thing of having both designations. Just as Tony Williams is saying keeping one system is more logical. I.e. metric first.

My 2 cents is to use the original name first and then the Metric.

By list metric first you would in perhaps 50% of the time disregard the history of the originating country / inventor.

That may well cause indexing problems, which could be solved by two indexes, this would make it easy for someone looking for say a 1.1" USN without knowing the metric measurements or needing a calculator to convert them. or looking through a long list of numbers to find the 1.1" designation.

When I say long list, look at Bob Hawkinson’s book.

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Not much more disregarding than has already been done with countless of cartridges ;-)

.50 BMG is 12,7x99 in Europe - .32 ACP is 7,65x17 SR, .380 ACP is 9x17, .45 ACP was (and sometimes still is) called 11,25 mm.

30-06 was designated 7,62x63 in several countries, the 5.0 in naval guns were just 127 mm in Norwegian service, 3.0 in guns were 76,2 mm, etc etc.

I do get your point, but it’s not a new concept/train of thought.

And again Ole is right.

Ole, not only Norway is having metric designations for inch-calibers but the whole NATO and the tiny rest of the world.

The inch designation matters only for pre 1945 calibers and as Tony has pointed out it its basically only the 1.1".
So we are talking about 1 caliber making a listing “backwards”. Isn’t the tail wagging the dog here?