I have a Core 15 that I bought 4-5 years ago, second hand from a guy that never fired it. Only thing I changed was the stock from a collapsible to fixed, because of my sate's laws, so now it has a rifle length buffer and spring.
Two range sessions ago my rifle was acting up. It was failing to feed. Sometimes it would fire, eject the spent round and go back into battery without picking up the next round. I took it home cleaned it real good and checked the gas block (when I first got the rifle the gas block was loose causing short stroking) I had it happen in past range sessions, but seemed to clear up when I stopped using certain magazines and I stayed using my Magpul ones. I was using .223 of various manufacture.
At mt last session I used 5.56 to see if the added pressure would fix it, but it was acting up again with all my magazines. I would fire 2 rounds and then it would fail to feed. Most of the time not picking up a round at all. I would pull the charging handle and get another 2-3 rounds before it would do it again. Then it did it again, but this time I couldn't pull the charging handle at all. It was stuck.
I took it home and tapped the charging handle with a rubber mallet and go it to eject. I tried running clean rounds into the chamber after I got it cleared and none of them would go in to full battery. The round would eject, but I had to pull real hard.
I later took to friend and tried the spent case that got stuck in mine, in his M&P and it went in and out flawlessly. I also tried my BCG in his upper and it worked fine. I tried his BCG in my upper and the case would get stuck in the chamber.
I have come to the conclusion it's a bad barrel, but before I throw money at the problem and find out it didn't fix it. I want to hear what some more experienced people have to say.