ammo for handguns

Question
what is the difference  between all the different mm in ammo for example 9mm 40mm etc. i really do not understand

Answer
That generally refers to approximately the diameter of the bullet.  So, .45 ACP  has a bullet diameter of approximartely 0.45 inches. 9mm has a bullet diameter of about 9mm (0.36 inches).  But some are fudged a little to sound better: Both .357 magnum and .38 special have bullets with a diameter of approximatly 0.357- 0.359 inches.

But it's different for shotguns: A 12 gauge shotgun has an inner diameter of the barrel that would just fit a round ball of a size that 12 of them made from lead would weigh one pound. The same theory works for shotguns of 16 gauge, 20 gauge, and 28 gauge.  But a .410 bore shotgun has a barrel bore diameter of about 0.410 inches.

Is that confusing enough?  But I didn't make the rules.

Hope this helps!

Dave