Why .22 instead of a quarter of an inch? Why not an even 5mm instead of 5.56mm? How did such odd calibers become standards?
5MM Remington Magnum was made for awhile. Didn’t sell well. There is a wildcat centerfire round, 5MM Craig that has been devolped so that old 5mm rimfire rifles can be converted into a reloadable cartridge. There are many, many calibres of ammunition. Some just sell better and stay more popular for such reasons as poor marketing, actual performance, lack of manufacturers of ammunition and arms to shoot it, the economy at the time introduced, etc.
http://gunbroker.com/Auction/ViewItem.asp?Item=49642833
There is a 5mm pellet for airguns as well. .20 calibre
http://www.airgunsbbguns.com/Benjamin_Sheridan_Cylindrical_p/bnj50.htm
It is not as easily found as the more popular .177 (4.5mm) and .22 (5.5mm) pellets however.
Just a W.A.G., but the odd sizes might be a legacy of the old musket days. Back in the 19th century ammunition was spherical (“ball”) and usually measured not by diameter but by weight: so many balls per pound of lead. (I believe “gauge” as in shotgun size is still based on this). Weight could be determined more accurately, and most firearms weren’t manufactured precisely enough for the exact diameter of the ball to matter much.
This all changed when first Minie balls and then cartridged rounds were introduced, along with breechloading. Now the exact diameter of the slug mattered to at least hundredth of an inch accuracy, sometimes more. Bullets and cartridges went through a period of standardization where not only the diameter but also the length (and therefore total weight) of the slug and the charge of propellent had to be optimized to each other. And no sooner was standardization achieved then black powder was replaced with more powerful chemical propellents like cordite, and the whole process had to be repeated.
So basically the odd sizes are “legacy” holdovers from the whole process. Add to this that conversion between metric and American measurement will give you odd numbers no matter what, and you get things like the 7.62 round.
What can make it more confusing is the lack of standardization in what is being measured. Is it the diameter of the bullet, the grooves of barrel or the lands of the barrel? Some designations are just rough approximations. You have to look in a reloading manual for the actual dimensions.
Much of the time, designers build the bullet and the barrel that they want and then try to descibe it with a number that may or may not accurately decribe its dimensions. That is really more sensible if you think about it. Most bullets and barrels are custom fit combos so there isn’t much need for them to be standardized or have bullets that are standardized sizes. There are some exceptions to that and a common one partially proves the point. .357 magnums will shoot .38 calibre ammunition just fine and the bullets are the same size even if they aren’t listed that way (a .38 will not shoot .357 ammo however).
I have a history of rifle development and the number of calibres that are made (and even still available in limited numbers) is staggering. Shooting is a precision art and gunsmiths and bullet reloaders have built any gun and bullet configuration that they could think of to fill a niche or squeeze out better performance. Only the very successful ones caught on and are commonly around. The rest were left to collectors or to deteriorate away.
They described their creations with a number that could be derived several different ways. The bullet itself was usually custom designed and it usually isn’t ideal to have stock bullets in incremental sizes to serve every purpose. There is crossover use of bullets in some types of different cartridges but there is little advantage of forcing their design to arbitrary numbers.