Can I fire .380 ACP ammo in my 9x18M Makarov?
NO NO NO NO NO !!! There are those that disagree with me including an author of an article in Handguns magazine and gun dealers who wants to sell you a Makarov. The 9x18M cartridge is not a "true" 9mm because it's really 9.2mm. Therefore the bullets are sized .364" and .365" (barrel sizes vary...slugging yours is best to find out your bore size). Yes, you can probably get away with it a few times, but accuracy will suffer, you'll get gas blow-by, your barrel will wear out faster and for what? Save a few pennies? Hardly. 9x18M ammo is usually cheaper since you can get it surplus. If you absolutely want to shoot .380 ACP ammo from your Makarov, then buy a .380 Makarov or a .380 replacement barrel.
For a car analogy, have you ever seen or owned a car that burned oil? How long did the engine last after it started blowing blue smoke? Probably not long. The reason is that the pistons and rings are now smaller than the cylinders and each explosion sends gasses by-passing the piston ("blow-by"). You're literally burning the sides of your pistons now and engine death is imminent. That's basically what you're doing by shooting undersized bullets.
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I have a .380 ACP pistol and want a little more power. Can I shoot 9x18M in it?
ABSOLUTELY NOT!!! This is even worse than the other way around. The bullet is too big (.364") to fit into the barrel (.356"), the cartridge will probably not even chamber, and even if it does, you may blow the gun and yourself up. Really, 9x18M Makarovs are not that expensive...just get one if you want to fire 9x18M. You can also change the barrel and you gun will now fire 9x18M. And if you want more power, get a .454 Casull.
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How about 9mm Parabellum? It fits in the chamber.
Holy smokes...what are you trying to do? Kill yourself?!? 9mm Parabellum is a very high pressure round intended for locked-breech pistols. Look, fire .380 ACP from a .380 ACP pistol. Fire 9x18 M from a 9x18 M pistol. Fire 9mm Parabellum from a 9mm pistol. It's that simple. If you want multi- calibers, get a .357 Magnum; then you can fire .38 Special from it.
If you're really hellbent on tickling the last bit of performance out of your Makarov, see the Accessories and Services page on how to lengthen the bore and make a 9.2x19 pistol. Also see the part about springs.
http://www.makarov.com/makfaq.html