I had the same issues with feeding in my AP4.  I found out a couple of things.  First, I ended up picking up some Magpul Pmags.  Right away, the bulk ammo I was shooting fed 100%.  I tried the supplied DPMS mags, and again, issues with feeding.  When I tried reloads, I had a few issues.  I found I really needed to "cam over" the press when sizing the cases.  That really helped, but the thing that did the trick was polishing the chamber.  I took a brass .410 bore brush attached to a cleaning rod and wrapped portions of  blue shop towels (the more expensive ones you can pick up at wally world in the automotive section) around to the bore brush.  Next, I applied a small amount of Flitz polishing compound and slowly polished the chamber to a glass like finish using my cordless drill.  It is amazing how much nicer the shells extract.
Also, about modifying your gun.  I purchased the AP4 from Lucas Tactical with a few upgrades.  I had the DPMS carbon fiber added to save some weight and free float it.  I added a Hogue grip (my favorite so far) and I added a Badger Tac Latch.  I later replaced that with the BCM Gunfighter charging handle (huge upgrade, highly recommended!).  I have a Magpul CTR stock which I really like.  I later swapped out the front sight base for a YHM flip-up front sight.  I did this when I had ADCO turn down the barrel under the handguard.  The handling is night and day difference.  Also highly recommended!