David has the basics down, but it can be confusing after that. For example, the 9mm and .380 are actually slightly smaller than the .38 special in bullet diameter. They use a .355 diameter bullet and the .38 is actually a .357 diameter.DJB.45acp wrote:I feel your pain, been there and had to learn them all over again.
First issue is that some calibers are given in diameter of bullet and some in millimeters.
Next issue is that case lengths differ greatly. And finally the case diameters can also be very different.
All that said, here is a short handgun caliber list that I'm sure will be added to.
Larger to smaller diameters:
.45 caliber = .45" diameter
.44
.41
.40 = 10mm
.380 = .38 = 9mm (they also call the .380 a 9mm short because of the case length)
.357
.32
.25
.22
And then the question is exactly what is "bigger"? For example, a .357 magnum has a smaller diameter than a .40 caliber but a lot more energy (read power). Many of the older revolver calibers have two versions, such as the .38 special and the .357 magnum. These two are the same diameter but the .357 case is slightly longer to allow for more powder and, thus, more power. other combinations that are similar are the .44 Special and .44 Magnum and the (auto, not revolver) .40S&W and 10mm.
For easy rules to remember, in general the diameter of the bullet is the name. If it is in English measurements, it is written with a decimal point and indicates the diameter in inches (.45 acp is just under one-half inch, .357 magnum is just over one-third, etc.). If it is in metric units, it is the diameter in millimeters and can be easily converted to inches if needed for comparison.
At least, this is my understanding, and there are some exceptions to any general rule.