Im looking at building a new AR. Im thinking of getting one in 9mm however I don't really know anything about 9mm ARs. I have 2 of them in .223 already. Id like to know whats different on the 9mm rifle, I know I need a 9mm upper but I have no clue what lower mods are needed. Can anyone school me on this?

You are in the wrong forum there is a rimfire and pistol caliber fourm that will have a FAQ with everything that you need try here  www.ar15.com/forums/forum.html?b=3&f=15.   That being said you will need a 9mm upper in either colt style or Olympic style which one you pic will influnce which mag block adapter you will have to buy.    Along with these parts you will need a 9mm hammer and a heavier buffer
(H buffer at min or a 9mm buffer)    9mm are blow back so you can choose whatever hand guard style you want without haveing to worrie about bolt bounce etc.

My current setup is a RRA lower with a RRA CAR upper HY-TECH mag block adapter.  You can have your bolt ramped and use a DPMS non notched hammer( reg 5.56 hammer) if you only have one lower and want to use it for more than one upper.  Hope this helps there is a ton of info in the other fourm with the pros and cons to the different systems.

I hear that the Olympic AR's in 9mm or .40 that take the Glock mags are the ones to get.  Inexpensive and available magazines and last round bolt hold open.  The only other pistol caliber AR that has the bolt hold open feature is the Colt/RRA, but the Colt mags are expensive and hard to come by.

Randall

Quoted: I hear that the Olympic AR's in 9mm or .40 that take the Glock mags are the ones to get.

There's NEVER an excuse to buy anything from Olympic arms.