Again, it's not that straightforward. The ATF has ruled that most common rifle rounds such as 5.56, 7.62 NATO, and 7.62x39 are actually considered pistol round

Again, it's not that straightforward. The ATF has ruled that most common rifle rounds such as 5.56, 7.62 NATO, and 7.62x39 are actually considered pistol rounds, and thus subject to AP regulations, because someone somewhere has detached the stock from their rifle, thus converting it to a "pistol".