Howdy
A 38 is not really a 38.
It goes way back to the mid 1800s. Early metallic cartridges often used what was called a heeled bullet in those days. The bullet was the same diameter as the outside of the case. A narrower diameter 'heel' at the base of the bullet was crimped inside the cartridge case. But the outside diameter of the bullet, that rode against the rifling of the barrel, was the same diameter as the case. So an early 38 really was a 38. By the way, if you want to see a heeled bullet today, look no further than 22 rimfire ammo. They are the only common cartridges still manufactured with heeled bullets. You will notice that the bullet is the same diameter as the case.
The drawback to this scheme was that the bullet was lubed with soft waxy lube that often picked up contamination in the form of dirt or pocket lint. One early cartridge of this type was a popular Smith and Wesson cartridge called the 44 S&W American cartridge. It employed a heeled bullet and really was a true 44 caliber cartridge. In the early 1870s S&W won a contract with the Russian government to supply them with revolvers. The Russians liked the S&W break top revolvers very much, but they did not like heeled bullets, because of the problem with the bullet lube picking up dirt. So they specified a modern type bullet, that was the same diameter as the inside of the cartridge case. That way, bullet lube could be carried in grooves cut into the side of the bullet. When the bullet was seated, the grooves were seated inside the case, and the lube was not able to pick up any dirt, it was protected from outside contamination. Starting to get the picture? The new S&W round was dubbed the 44 Russian, and it was very popular with target shooters for many years. But even though it was called a 44, because the case diameter was 44, the actual bullet diameter was .429, in order to be inserted inside the case. In 1908, S&W lengthened the case by about 1/8" and called the new cartridge the 44 Special. About 1955 it was lengthened again and stuffed with more powder and called the 44 Magnum. But all three cartridges used a .429 bullet.
Same thing with the 38 Special. By the time it had been developed in 1899, inside lubed bullets were common. True bullet diameter was .357 or .358, depending. But it was still called a 38, just out of habit, and because that was the diameter of the outside of the case. In the early 1930s S&W was experimenting with some high powered, high velocity 38s that could only be fired in heavy framed revolvers. If they were fired in a standard 38 they would probably have blown it up. So in 1935, S&W lengthened the case by about 1/10", so that the new round could not be chambered in a conventional 38 caliber revolver. They decided to call the new round the 357 Magnum, using the true bullet diameter in the name. The Magnum term was used because the president of S&W at the time was a wine connoisseur, and a Magnum was a great big bottle of wine.
By the way, don't get too hung up on cartridge names, they seldom tell the truth. Both the 30-06 and the 308 use .308 diameter bullets, don't they? Don't look for any logic in cartridge naming conventions, there isn't any.
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