Uzi Magazines can be easily modified to work with the Colt Style 9mm upper. That is, you need to mill out a magazine catch on the correct position, as well as sometimes needing to mill down the back of the magazine for bolt clearance. Once modified, the magazine can easily be used in either the AR 9mm or Uzi. It doesn't adversely effect the magazine.
It's pretty common in the 9mm AR world to use cheap bulk Uzi Mags to feed "Colt Style" ar's. (Includes Rock River Arms)
The "Other" type is the single feed magazine. They tend to use Sten Mags for cheap feeding. (Olympic Arms)
Both the Colt style and Olympic arms style AR 9mm conversions can be had with a Mag Well Insert to use the correct 9mm magazines.
There is a THIRD type that is coming around using GLOCK 18 Magazines. So far Olympic Arms makes a complete rifle (Will not sell just the lower) and Lone Wolf Distributers is now making a Glock Only lower. These can not easily be converted from one caliber to the other. (The magazine catch on the Glock Magazines is not easily compatible with the AR magazine relase.)
There is a company that makes magazine wells SEPARATE from their lowers, but I can't recall the name off the top of my head.
I have a couple Lone Wolf Dist. Glock only Lowers on back order. I plan to build a couple dedicated 9mm Rifles for multi gun matches and plinking.
As far as the magazines being "difficult" to extract, that's not unusual. The Mag Block usually is a bit tight and will loosen up a bit with use.
