>The only thing I think we can conclude about the incoherence of firearms legislation is that they have staunch and effective opposition from firearms advocates. I do not think it is at all valid to look at them and say "legislators don't know what an assault rifle is".
Well, if we're calling AR-15s and centerfire semi-automatic rifles with detachable magazines assault rifles, then we don't know what assault rifles are. The term has been in use since WW2 and refers to selective fire rifles with intermediate cartridges and detachable magazines.
I'm not entirely sure what you mean about the original 1980s proposals - afaik the only major 1980s attempts at legislation occurred after a school shooting in California, had limited opposition in California, and was enacted into law. It included classifying rifles as assault weapons if they had detachable magazines plus at least one of other relatively features, such as pistol grips, barrel shrouds, folding/telescoping stocks, etc. Most of those features have limited to no impact on the ability of the rifle to be a deadly weapon.
>(The AR-15 itself, of course, was designed by Eugene Stoner to be a standard-issue US military rifle.)
The ArmaLite AR-15 designed for the military was a select fire rifle that ultimately became the M16. The trademark was sold to Colt. Colt later used that trademark in producing the semi-automatic civilian version. An AR-15 or AR-15 clone a civilian can buy is not the same AR-15 that Eugene Stoner designed.
As best as I can tell, most legislators have no idea what assault weapons are, what assault rifles are, or much at all about guns in general. And this includes those on the conservative side. I'm pro gun control, if we can find a way to effectively do it, but if we're actually concerned about human life, assault weapons don't even make sense to be the focus. Over 60% of homicides committed with firearms are done with handguns. Rifles only make up about 4%. Mass shootings are scary, and make news, and are terrifying. But they're responsible for a tiny fraction of gun related homicides, and the focus on rifles (and particularly features that are more related to appearance than efficacy in being deadly weapons) screams to me that it is almost entirely a political play and less related to any real concern over the healthy and safety of those living in America.