Is there a difference ??

[JJW]

A newbie asks:
I have seen 9 mm. and 9x19, and 9x21, shown as a gun's caliber. What is the difference ? Are they interchangeable ?

 

[Treo]

They are not inter changeable.

The X what ever refers to a case length

 

[Black Dragon]

The 9 before the X is the diameter of the bullet itself in millimeters(not the case).
The 19 or 21 after the X is the length of the case in millimeters(not including the bullet).

Some bullets are measured in mm some are measured in caliber.
mm = Imperial measurement
caliber = American measurement

So a 9mm is 9 millimeters in diameter
and a .40 cal is .40 of an inch or 3/8" in diameter

 

[Phillip Gain]

What Treo said. BUT...

It's worth mentioning that "9mm" typically refers to 9x19mm. Which is also referred to as "9mm Para," "9mm Parabellum," and "9mm Luger." These are all the same cartridge - this is a VERY common caliber.

9x18mm is also referred to as "9mm Makarov" - this is used in mostly Soviet, Hungarian, and Bulgarian pistols, many of which were imported in the 1990s.

9x17mm is the same cartridge as .380ACP. Very common cartridge. It's almost always sold in the US as .380 though, almost never as "9x17mm"

9x20mm is the "Browning Long" cartridge and is obsolete. If you have a vintage Browning pistol that uses this caliber, you're outta luck for cartridges, and will need to learn to custom cut your brass cases and do your own reloading, if you want to shoot the thing.

 

[Purple]

What Phillip said...and... 9mm Kurz is really .380 ACP. The Kurz is German for Short. Also the 9mm Browning, 9mm Corto, 9mm Short, and 9×17mm are all .380 ACP'.s

 

[hyundailarry]

I understand the confusion there are so many available. But, and I not trying to be a smarta.. but have you tried GOOGLE they can teach you everything you could possibly want to know. There is more there than you will ever get on here. Good reading.