Did you clean and lube your M4 before you shot it. I took mine apart and cleaned it, lubed it with the provided oil, and reassembled it. I started out with 5 3" #4's, that was all the heavy rounds I had, then I put 100 Federal low brass #8's from Walmart thru it and never had a failure of any kind. I even did a few quick as I can mag dumps and had no problems. I'm up to 350 rounds now and it has been perfect. I don't know if i've just been lucky or if the liberal lube on the bolt group is the difference. I hope you can get yours to run right. They are a lot of fun to shoot when they run good.