How is it possible to fire .38 Special rounds out of a .357 Magnum weapon? Wouldn't a .357 barrel be too narrow for a .38?
No, it's the same bore diameter.
The reason is that different places measure the actual bore diameter differently, depending on who makes the firearm, who makes the ammo, etc etc. It's all very confusing...
I'm a firearms regulation policy officer, and trying to explain this to law writers and politicians (who have no idea about firearms generally) is a nightmare...
Basically: a calibre can be measured from land to land (the rifling ridges in a barrel), or from groove to groove (the gaps between the rifling), or from groove to land.
As well, it can be represented in 'imperial' measurement: either as .357 (of an inch), or a gauge (generally only for shotguns). A gauge is the number of lead balls of a particular size that make up a pound in weight - so 12 guage is larger than 20 gauge.
It can also be represented in metric, such as 5.56 mm (which is .223 cal in imperial).
On top of all this, the size of the projectile, the size of the cartridge case, etc also determine the type of ammunition, which is why you get numbers like 7.62 mm NATO and 7.62 x 39 (soviet).
These are just conventions that have occurred over 700 years of firearms use in the western world. Not the best, makes no sense, but it will never change.