whats the difference between 9x21mm IMI and 9mm Parabellum? they must be different in some way? or am i wrong?

My Google-Fu implies that 9x21 is largely a legal construct to get around certain laws about civilians owning weapons chambered in 'military calibers', or some similary stupidity.

There are a few references to being able to hand-load 9x21 a little bit hotter than 9x19 but probably not by much, the chamber pressure limits of 9x21 are probably the same as 9x19.

Such laws exist in Italy and a few other European countries where various backwards and largely impotent gun control laws have created an artifical market for stuff like 9x21mm and .380ACP. 9x21mm is going to hit slightly harder than 9x19mm (otherwise know as 9mm, 9mm Luger, and 9mm Parabellum) for obvious reasons (2mm of extra metal or more propellant), but, overall, differences in ballistic performance are supposedly negligable and academic at best.

Looks like Arethusa and Crusher are on the money.

I took a look at the commercial load variations and found a Muzzle Energy increase of about 11% comparing the most powerful loads of each.

It sounds like a 9x21mm firearm could fire both rounds then, if the total dimensions are similar.

I would think that if the intent is to not be able to use military ammo, they would want 9x17mm or 9x18mm, not 9x21mm.

yes that's kind of like the argument that one could fire .40s&w (10x21mm) in a 10x25mm gun, there's a 4mm difference and most of the time the extractor will catch the case rim and hold it in it's proper place in the back of the chamber up against the firing pin. but what about the few times that the extractor doesn't catch the case rim and it goes to the front of the chamber so the firing pin either doesn't make contact with the primer and gets damaged by dry-firing or gets very badly damaged as the firing pin ignites the primer and the rest of the powder which causes the case to go slamming back against the firing pin. so is it worth trying it? probably not unless you got a lot of cheap .40 or 9x19mm that you can't use otherwise and a lot of spare extractors and firing pins.

anyway if they were to make a cartridge with less case as a legal replacement to 9x19mm it would mean the cartridge pressures would increase a lot and 9mm is already a fairly high-pressure cartridge. it would mean a lot of guns created for 9x19mm wouldn't be strong enough to handle the civilian replacement 9x17 or 9x18 or whatever. you might want to take a look at the Glock 36 versus the Glock 37 for an example of this.

so, by making the round larger and able to pack more powder behind the bullet, it becomes legal because it's not "law enforcement or military". this is what happens when you let idiots make laws.

What otomik said.