Are the AR's limited to single shot in the US? Is it possible to to modify them back to fully automatic?
Are AK's also modified for single shot use?
Andrew, if by "single shot" you mean "one shot per trigger pull", then yes, at least insofar as civilian (NFA Title 1) guns go, and no, they are built on civilian receivers that are designed to make them difficult to convert to full auto.
Under U.S. Federal law, all guns capable of firing more than one shot per trigger pull, as well as all guns that can be easily converted to do so, are subject to the very strict Title 2 provisions of the National Firearms Act of 1934 as amended in 1986, instead of the Title 1 provisions that apply to ordinary civilian guns. Title 1 guns include deer rifles, squirrel rifles, AR-15's, civilian AK lookalikes, etc. Restricted Title 2 weapons include actual select-fire M16's/M4's, actual select-fire AK's, howitzers, M203's, RPG's, sound suppressors, short-barreled rifles and shotguns, and so on.
In a nutshell, it is a 10-year Federal felony to possess any Title 2 weapon in the United States outside of police/military duty or government service, unless you first obtain special authorization from the BATFE (for automatic weapons, it's a Form 4), which is technically a tax stamp but in practice works like an authorization form. This the same level of control exercised over things like 105mm howitzers, artillery shells, grenades, shoulder-fired rockets, and the like, and involves a more extensive application process than an ordinary gun purchase. In addition, all automatic weapons manufactured after 1986 are strictly limited to police/military/government and their suppliers, which means that actual Title 2 AR's and AK's are rare collector's pieces (I've seen prices of $15,000-$17,000 for an M16 or an actual civilian-transferable pre-1986 AK-47).
FWIW, any gun easily converted to full auto *is* a full auto for the purposes of Federal law even if not actually converted, and is therefore subject to the Title 2 restrictions and the 1986 ban.
The U.S. "assault weapons" ban 1994-2004 affected *only* non-automatic Title 1 civilian guns, and the *only* difference between a banned gun and a non-banned gun was configuration. An AR-15 or a civilian AK with a smooth muzzle or a pinned-on brake was not an "assault weapon" under the law, but a Ruger mini-14 or 10/22 was if you fitted it with a stock that folds for storage. It had nothing to do with "military" origin, rate of fire, lethality, or use in crime, and everything to do with cosmetics and ergonomics. Under the law, a newly manufactured rifle could have a protruding handgrip OR a flash suppressor/flame damper OR an adjustable stock, but could not have more than one such feature.
The irony of the whole brouhaha is that rifles of any type are consistently the least misused class of firearm in the United States, accounting for only about 2.5% of U.S. homicides. Many U.S. states report zero rifle homicides in any given year.
As to the broader question in the OP, yes, using the term "assault weapon" to refer to civilian rifles is ignorant. The term is loaded, and so elastic as to be meaningless (under the 1994 definition, a SAR-1 7.62x39mm AK wasn't an "assault weapon", but a Ruger 10/22 with a Butler Creek stock was).
