Bartholomew Roberts
Member
(Call me cynical, but I suspect that the civil rights movement in the 1960s alerted the anti gun crowd to the fact that demonizing "Saturday night specials" was not going to go over, forcing them to move to the funny looking EBR as the new scapegoat.)
Actually, the original push for gun control was to ban handguns. Handguns were actually a part of the originally proposed NFA (which is why short-barrelled rifles and shotguns were also part of NFA). This got no political traction and handguns were removed from NFA. Although late gun control (like the 1968 GCA) would target imported handguns via the points system and in general do everything to make sure handguns remained expensive and out of reach. The Brady Campaign was previously Handgun Control, Inc., who was previously the National Coalition to Ban Handguns.
Somewhere in the late 1980s, Violence Policy Center came up with the idea of targeting semi-automatic firearms because it would be easy to associate them with machineguns and bans on handguns were rapidly losing political support. BenEzra published a great chart on this in another thread; but it boils down to gun banners will ban anything they think they can target.