Hmm I've reassembled the gun a few times and had this problem before, I've forgotten how I fixed it but for some reason right after I pull the charging handle over the notch it gets stuck right before it can slot in.
Any ideas of whats blocking the Upper to attaching into the lower
http://i.imgur.com/efvyzNx.jpg
http://i.imgur.com/hEJDEJJ.jpg
http://i.imgur.com/7fxTxMp.jpg
Any ideas of whats blocking the Upper to attaching into the lower
http://i.imgur.com/efvyzNx.jpg
http://i.imgur.com/hEJDEJJ.jpg
http://i.imgur.com/7fxTxMp.jpg