And the mental masturbation thing. Sorry!
What I meant to say is if an AR15 drop in auto sear does not fit a AR10 or you can't find a crafty gun smith that can make it fit and work properly. Then, this hole thing is a day dream (thus, the mental spank the monkey!), because it can't happen. Auto sear this the only way to convert the gun legally. There are no existing AR10 registered receivers and no way to make a completely new machine-gun. Only other option is becoming a Class III manufacturer and although fairly easy to do. It is also very expensive to maintain.