Out of curiosity, why aren’t firearms and artillery typically standardized more to something convenient and tidy like 5mm, 10mm, 100mm, 125mm and 150mm?
Since almost all of them originate from the old imperial measurements.
5.56 is basically a 0.223
7.62: 0.308
155mm: 6inches
203mm: 8 inches
ETA: When you read about WW1 and WW2 era non-anglo warships having guns of a certain size, note that they were often metric translated to the nearest inch.
For instance Bismarck’s main battery was 380mm, which is 14.96 inches but always expressed as '15
Yamato was 460 which is 18.11 inches but is usually written as '18.
I’m not seeing how that explains anything, since those are equally non-round numbers in imperial. And they aren’t quite right anyway.
5.56 is 0.219 inches
7.62 is exactly 0.3 inches, so where did the caliber .308 come from?
In this particular case its since each fire from a rifled projectile. The US measures bore diameter between groove (the depressions in rifling) and a 0.30 bullet has to be approximately 0.308 inches if it wants a good gas seal.
My WAG: I think that is what happens if you start with a standard rod, machine it and drill the biggest possible hole in it.
i.e. You start designing with the outside diameter of the barrel.
…maybe?
Consider a cartridge like 5.7x28mm. 5.7mm seems like a more round number. However, this is the diameter of the bullet, same as the “5.56mm” bullet with the same diameter. Similarly for 9mm (Wikipedia says 9.01mm).
So there is inconsistent nomenclature.
I have never tried designing weapons, but starting from scratch I may not specify the ultimate exact diameter in advance—it might be dictated by optimizing other factors.
Thanks, AK84.
I would think that the general diameter of the bullet and size of the cartridge has to do with the purpose of the cartridge, like the P-90 being a personal defense weapon that can shoot through Russian paratrooper armor.
Well yeah… the cartridges generally are sized appropriately to their intended use. For example, when cartridges came into vogue, rifles and pistols still fired relatively large and heavy bullets at relatively low velocities.
When smokeless powder came into use, it seems like there was a shift almost immediately to much smaller diameter bullets with much higher velocities, probably because they had similar energy, but much longer ranges. For example, the famous “Trapdoor Springfield” of 1873 had sights that had gradations to 1100 yards, while the M1903 Springfield had sights marked out to 2700 yards.
Of course with the advent of the machine gun, the long-range role fell to them rather than your average infantryman, and research after WWI and WWII found that the vast majority of infantry combat occurs under 300 meters. So bore diameters and cartridge lengths went down, as there wasn’t any need for infantry rifles capable of reaching out to 2500 meters anymore. In many armies this means that they have two types of ammunition- the older, larger type (7.62) for machine guns expected to engage in longer-range fire, and the smaller bore type for rifles and carbines (5.56/5.45) meant to be wielded by individual infantrymen. (interestingly enough the Russians still use the same old cartridge from 1891- the 7.62x54R in that role).
We’ve seen a further instance of that phenomenon with the recent personal defense weapons- the 5.7x28 and the 4.6x30 are the prime examples. Not necessarily smaller diameters, but certainly shorter cartridges with shorter ranges, etc… because they’re expected to be very close range weapons meant to replace pistols and submachine guns, but with greater effectiveness against body-armor wearing enemies.
The nominal size was probably a case of simplification- why call it a .308 if you can call it a .30 cal and everyone knows what you’re talking about? Same with 9mm vs. 9.01mm.
A lot of it was trial and error. You have lots of gunmakers constantly testing and manufacturing lots or ammo of different sizes and shape. The ones we know today were the most successful ones, or at least the most popular. Why should the best round have a caliber in a round number?
You mean .303
Surprisingly not a line invented for the movie, Morant did reply at his court martial that
“Was it like this? No; it was not quite so handsome. As to rules and regulations, we had no Red Book, and knew nothing about them. We were out fighting the Boers, not sitting comfortably behind barb-wire entanglements; we got them and shot them under Rule 303!”
7.62mm is literally .30"
There are 10mm bullets.
5.56 mm is 7/32 inches.
Around the turn of the century (ish), most militaries settled on something close to .30 cal. for their rifles, as that had plenty of “stopping power” without being too over-powered and difficult to control, and performed well at long ranges. 7.62 mm is .30 cal.
The French wanted something between 14 and 16 cm. I’m not quite sure how they settled on 15.5 cm (aka 155 mm) but it probably is worth noting that it’s fairly close to 6 inches.
203 mm is 8 inches.
A lot of weird caliber rounds are a way of differentiating between two very similar sized (at least in diameter) cartridges. For example, .38 special and .357 magnum rounds both actually have the same diameter, but they intentionally call them two different things just for easy differentiation.
It should also be noted that .357 inches is basically 9 mm.
5.56 is NATO.
There is a 7.62x51mm NATO cartridge (7.82mm diameter bullet)
in service 1954–present, derived from .30 inch calibre cartridges
Specifically, 5.56x45mm.
As is 7.62x51mm.
But not 7.62x39mm.
https://en.wikipedia.org/wiki/7.62×39mm
Wikipedia list of approximately 5mm caliber ammuntion:
Wikipedia list of 7.62mm caliber ammuntion:
A common thread that you see in the various descriptions and histories is that the modern metric caliber rounds started out based on or specified in decimal inches (.22 caliber for 5.56, .30 caliber for 7.62). Both .22 and .30 were very common bullet types at the time these rounds were specified, and when designing almost anything you start with what you know.
Now a round 1/3" bullet would be about 8.47mm, but this exact measurement seems to have fallen out of fashion for some reason, at least according to the Wikipedia list of current 8mm rifle cartridges.
Which are good points – as you work out the ballistics of the round itself and of the round + weapon to be used in, if you are starting from a blank page then there is no reason to first set in stone a “nice round number” for the caliber, so you end up with whatever you end up as your optimized result and it’s up to the arsenal’s PR department if they want to round up/down; while if starting from improvement on what you already have, you will probably be making some tweaks on the prior design and give it a designation based on the changes and that distinguishes it for stocking purposes. You don’t really want two incompatible lots of ammo both labeled “ammunition, rifle, .30 cal ball” in the warehouse.(*)
(* For instance the 7.62x51mm NATO, which replaced both the American .30-06 and the British .303, mostly because even c. 1950 some sticks in the mud at the US Ordnance office insisted the alliance’s rifle ammo had to be something in or around “full power .30”.)
If you’re designing a round, and stuff like ballistic properties and ease of machining are considerations, why should “whether it’s a round number” even make the top 20 of your concerns?
Because military logistics organizations are fantastically conservative. Sometimes for good reasons, like reducing the prolifieration of multiple incompatible ammo types. Sometimes for dumb bureaucratic reasons, which often boil down to “that’s now how we’ve always done it.”
It has nothing to do with redundant/incompatible ammo types. You could standardize around any caliber whether it’s 7mm or 7.3218154mm. Also, just being the same caliber doesn’t mean that they’re the same rounds or that they’re compatible - consider 7.62x39 7.62x51 and 7.62x54r.
Trying to make something more aesthetically appealing or quicker to say because it comes out to a round number is a silly reason to drive military design. You could always shorten rounds with nicknames if you want, like how 7.92x57 is “8mm mauser” or 8mm.