Been there, done that. Short barreled AR's are VERY finicky. Everything has to be just right, and will benefit from opening up the gas port just a LITTLE. But the first thing to do is make sure the chamber is polished perfectly smooth. If you have a chromed chamber use scotch-brite wrapped around an improvised drill bit and polish it as close to mirror finish as you can get without getting past the chrome, get rid of the tool marks. If you have a stainless or alloy bbl. use 400 grit wrapped around the scotch-brite to get a mirror finish. The polished chamber has usually solved all of my extraction problems. Another issue is the extractor pressure, you need to install a black or red extractor spring or "defender" type extractor stiffener, or you can use a small "O-ring" of appropriate size. Short barreld AR's cause extreme "extractor bounce" letting go of the round before it is out of the ejection port. Try the extractor fix and polish the chamber first, and I bet the problems will dissappear. Also check your mag alignment with the feed ramps, as long as it "dry cycles" the round into the chamber by using the charging handle, you should be o.k. If not, you may need to dremel your feed ramps, I.E. m4 type ramps. This is not very hard, just follow the countour of the current ramps and bring the ramp lower into the mag well, a little at a time. I have had to actually dremel into the upper receiver just a bit to compensate for mis-aligned parts. Just do a little at a time, until the rounds cycle reliably into the chamber without hitting and jamming up against the barrel extension.