Honestly, as someone who associates with a lot of gun enthusiasts, I think that the AR platform is quite easily modified into fully automatic -- just replace the sear from a semi-automatic sear to a full auto sear.<p>You have to have a BATFE approval and pay a tax stamp to possess a full auto sear, but the work in switching from semi to full auto is pretty minimal.<p>A more apt scenario that people would clamor behind is a semi-automatic firearm that had a fully automatic failure state.  That would be terribly unsafe, and everyone on both sides of the gun debate would be demanding it got fixed, just as they would if a firearm were on the market that could accidentally fire when dropped.<p>Both of those used to be fairly common scenarios, but are now both exceedingly rare thanks to advancements in gun safety.