It was always intended as a method to dry up the inventory of such weapons, make them increasingly rare so that even criminals would have a hard time getting their hands on a fully automatic weapon.
How'd that work out though? They just buy stolen military grade weapons on the black market, or smuggled from other countries (or maybe even our own government running illegal guns and "losing track" of them. Oops... I guess they didn't count on criminals breaking the law to get what they're after.
I think that “they” always knew that criminals would get their hands on whatever they wanted to. After all, if you have the logistical capability to get in hundreds of thousands of bales of marijuana into the country, you could simply put a gun (full auto or otherwise) packaged in a sealed plastic bag inside of each bale - no new logistics required, just a bit of extra weight for all of the profit you’d get for bringing in untraceable guns.
My opinion is that Richard Hughes just hated gun owners with all of his cold, black heart, and used the opportunity (on a voice vote which, on tape, seems to LOSE) to attach a rider to the ‘86 FOPA to forever ban new full autos. Just to screw gun owners, and just to ensure that the citizenry would ALWAYS be inferior to the government. I’d like to piss on his grave every day, but he isn’t worth the trip.