I know it did not work out well for the civilian contractor on Ft. Hood. He charged the shooter with a chair. He was shot, and killed. If I remeber correctly the shooter shot him in the head.
A disarm worked in the Kip Kinkle (sp?) school shooting. Kip Kinkle killed his parrents. He then took a 10/22 rifle, and a hand gun put them in the family car. Drove to school. Walked in, jumped on a table, and started shooting people with the rifle. A teacher jumped on the table, grabbed the rifle. He managed to get it free of the shooter's hands.
Myslef I would probably go for cover. If I had the elements of concealment, and surprise working in my favor I would possibly try a disarm, and take down.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.