I just got my first AR15.  Since I live in CA I am forced to get a FAB 10 lower reciever.  When I have the fixed 10rnd mag filled the upper reciever will not close all the way without using a lot of force.  And when it does it is impossible to pull back on the charging handle.  If I load the magazine with 9 or less rounds the gun functions flawlessly.  Its as if the mag will not allow the upper to compress the rounds in the mag at all(if its supposed to do that).  What do you guys think my problem is?  I will not continue to just load 9 or less rounds its too many as is.  What do you think I could do with out having to send it back to the manufacture?  

Thanks for your time and help I really appreciate it.

- William