If you've got questions, just ask them here
This was already covered, but what the heck:
"What’s the difference between a .380 and a .38?"
.380, as stated, is meant for use in automatics, while .38 is for revolvers. Both names are misleading, because neither round has a diameter of .38
"What’s so special about a .38 special? "
Nothing. For some reason it's really popular, but I don't think it's all that great. I'd much rather have a .357, about the only gun that would be better in .38 is the super light 5 shot revolvers. One of those in .357 would kick pretty hard, I think. I've never shot one so I don't know for sure...
"Can you really shoot .38 ammo out of .357 Magnum guns?"
Yes, but I hear that shooting .38s out of a .357 rifle doens't always work. Again, I've never done it, so I don't know for sure
"What does the ACP mean after .45 ACP? How’s this different than a .45?"
Auto Colt Pistol. I do see the apeal of .45 ACP, it has a good repuation as a "man stopper". It is more "specal" than .38 if you ask me...
Anyway, .45 ACP isn't the only caliber with an "ACP" in the name. There are .25, .32, and .380 ACPs. The "ACP" in the name helps you tell the differance better auto and revolver rounds. For example, .32 S&W is a revolver round, and .32 ACP is a auto round. Both are .32's, but they can't be fired from the same guns.
Why do I almost never hear of a .38 semi-automatic or a 9mm revolver?
Again, as said above, it is due to the rims of the cases. You pretty much have to have rimmed cases if it's going to work in a revlover...
And you didn't ask about this, but you should proably know so I'll talk about it. In other countrys, you'll see calibers with 2 numbers, for example, 9X18. The first number is the diameter of the bullet, in mm, the second number is the length of the casing. .380 ACP and 9X17 are the same round, and 9mm Luger is the same as 9X19. This also goes for rifle rounds. 7.62 X39 for example, is what the AK and SKS shoot. And then sometimes they add letters onto the end, for example, 7.62 X 54 R. I think the "R" means "rimmed", this is what the Nagant bolt action rifle shoots. In case you were wondering, 7.62 mm = .30 caliber