Grain is a unit of weight (or mass if you want to be pedantic). It is 1/7000 of an avoirdupois pound (the kind we normally use for weight).
Both bullets and powder are measured in grains. Pistol bullets typically are between 100 and 250 grains. The powder charge is 2 to 5 grains. There are many types and brands of powder with different performance characteristics.
All this stuff is as complicated as rocket science.
You can learn a lot by poking around http://www.ammoguide.com/. Don't hesitate to register. They might send an e-mail a month, but they're not spammers.
Wildscar wrote:Stupid Noob Here. I thought the grain had something to do with the power in the round.
Not a stupid question at all.
Both bullet weight and powder charges are measured in grains. The weight of the bullet (115 gr, 124 gr,etc.) is what you will see on the box in the store.
+p or +p+ ammunition will have more powder by weight, hence a bigger bang and higher velocity at the barrel.
More velocity and heavier bullet weight means more stopping power.
Clear as mud now, right!
Edited: I see seamusTX beat me to it....he must type faster than me.
"Superior firepower is an invaluable tool when entering into negotiations." - G. Patton
The most common (in the USA) 9mm bullet weighs 115 grains. These shoot well in my pistols, and usually velocity is a bit higher than heavier bullets, useful when hoping for expansion from a JHP. Good choice.
124 grain bullets are also quite popular - I believe this is the standard weight for military (NATO) ammo. Velocity is down a little bit from 115s, but is still usually enough to expand a hollow point. Another good choice.
Winchester's 127 grain may be unique.
Then we get to heavier bullets such as the 147 . . . these are usually subsonic, as high velocity is more difficult to obtain in a 9mm case without exceeding pressure limits. By most accounts, the terminal performance of these rounds is inferior to the best 115 - 127 grain ammo. These are often the round of choice in suppressed weapons.
Projectiles weighing less than 115 grains have fallen out of favor due to reliability issues (shorter rounds don't feed well in some pistols) but some of the specialty rounds - such as the 80 grain Glaser Safety Slug - have their devotees.
Powder charges will typically be in the range of 4 to 8 grains - a fraction of bullet weight - but note there are MANY different powders in use, with different burning rates, and switching powders DOES usually require switching charge weights in order to maintain proper chamber pressure.
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I don't carry 9mm that often (preferring .45), but lately my 9mm has held Corbon's 124-grain +P hollowpoint. Feeds reliably, and is controllable.
Once I get my reloading bench cleaned up, I'll load up some 124-grain Remington Golden Sabers and Hornady XTPs. Those are likely to take over as my "carry load."
In .45, I'll use either Corbon's 185-grain +P offering, or Hornady's 200-grain +P XTP. Recoil is snappish (moreso with the Corbon), but manageable out of a 3" barrel, although I wouldn't want to shoot more than a magazine or three at one sitting. Kinda stings the hands.
I thought the grain had something to do with the power in the round. 
