As stated the .38 Special and .38 S&W were named for the exterior brass at the mouth, an old hold over from when centerfire rounds were heeled bullets (Like the .41 Long). The same goes for the .44 caliber and the .32 series (.32 ACP, .32 S&W, .32 Long, and H&R and .327 Fed). Now some, like the .327 Fed are a mix of the old designation plus a nominal change to make the name noticeable. Some are named just to be noticed like the .218 Bee using a .224" bullet and the .454 Casull using a .452" bullet. I'm pretty sure the .454 was a reference to the 454 in3 engine for those of us old enough to be motorheads (but I don't know that for sure).