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There may be a simple answer to this but I've never understood, or just one of those dumb questions
Why do cartridge names not match the actual cartridge dimensions? the bullet itself is usually .2mm larger
for instance 5.56×45mm is actual a 5.7mm bullet, 6.5×55mm is a 6.7mm and 7mm rem mag is 7.2mm bullet
to do with how its measured. a rifle barrel is a tube with grooves cut into it. The grooves are called, well grooves, and the high points (where no groove is cut) are called the lands. So the bullet diameter can be a measure of the lands or the grooves of the rifle barrel.
Commercialism has a lot to do with naming as well. The Yanks tradionally didn't warm to metric names and in fact going back a bit there are a couple of cartridges that failed to take off in USA because they were given metric names instead of Imperial sizing names.
Have a read of this - https://www.thoughtco.com/how-ammuni...-named-1926781
And as Danger Mouse points out where the measurement is taken has influenced naming as well.
The classics are 303British versus 308Winchester with the 303B firing the larger diameter projectile.
Cheers guys, The high - low points of the rifling explains the .2mm and @zimmer that confirms my thoughts that it is just a bit chaotic and people name shit whatever they want forgoing logic
A lot is simply advertising / marketing.
.44 Magnum sounds a lot better than .429 Magnum. .38 Special rolls off the tongue easy too.
106mm anti-tank recoilless rifles are actually 105mm, but called 106 to prevent confusion with 105mm Howitzer or 105mm Tank.
Some names come from the original loading, like .30-30 was a .30 caliber cartridge originally loaded with 30 grains of powder.
.30-06 came from the year of introduction, 1906. The 3000 in 250-3000 Savage refers to the velocity, the first American cartridge to break 3000fps.
As a general rule the Europeans / English use the Bore diameter and the Americans the groove diameter there are some odd ones out there on both sides
if in doubt slug the barrel and do a chamber cast, if you stick to the common calibres you should be fine