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!