The receiver on the slug model is different. It has a metal strip under the receiver like the SBE 2 and super nova, which makes it hold tighter. I thought that Roberts didn't drill and tap the m2 20? I'm not saying your wrong. I just didn't want to take a chance on a 1400 dollar gun. I did read an article that a guy did have a scope fly off of an m2 after some shooting, he stated that there was not enough threads holding it.