long story short, a few weeks ago i got a AB-10 (last tec-9 version made by intratec) and have put close to 1000 rounds though it already and the only malfunction the gun has had was user error (my buddy didnt understand you simply let the bolt go when loading it)....ANYWAYS...I have really gotten on this "smg" kick and am currently looking at building a mac but have several questions...I have found cobray receivers already and plenty of different parts kits. my questions are..
-how do i know which part kit to buy?(am currently looking at parts kits from ftfindustries.com)
-can i get a m11/380 "upper" and use it and 380 mags in a 9mm cobray lower frame?
-can i use a open bolt fire bolt and parts kit in the cobray lower frame?
-what mag does the cobray lower frame use?

I have searched and read a lot, but still cannot find the answers to these questions, and the only way to learn is ask..  thanks for the help!