The AR-15 was prohibited in the United States under the U.S. assault weapons ban. The only reason it didn't make it to the prohibited list was that there was advocacy by the Dominion of Canada Rifle Association, which wanted to be able to use it for target shooting.
I did a report on this. It's 10 years old now, but if you look at most industrialized countries, they do not let civilians have the AR-15 because it's generally recognized as having enough characteristics of military weapons that most countries don't let civilians own them at all.