I need advice on a new Bushmaster XM15 E2 20 AR15 jam that occured over the weekend. While shooting a match and changing magazines I let my bolt close as usual. When I pulled my trigger, nothing happened. I tried to clear the round from the chamber but could not open the bolt by pulling the charging handle back. The range armorer had to push the bolt back with a screw driver through the ejection port. the round then ejected and the gun worked fine after that. I measured the round in question with a Dillon (go / no go) gauge and calipers checking all dimensions, and could find no problems with the round that was jammed in my gun. My gun was clean and lubed too. I can't figure out what happened... Any Ideas ?