It has been confirmed in many reports and by PD that it was a Ruger 10/22 with a 25 round magazine. It is not debatable.
http://us.cnn.com/2016/09/26/us/wash...ing/index.html
http://www.nbcnews.com/news/us-news/...murder-n654586
http://www.cbsnews.com/news/washingt...sed-documents/