As to the cops knowing the difference; unfortunately many Chiefs are politicians and gun grabbers. They know but lie. I've seen Brady Bunch produced videos of cops using full auto rifles (M-16's) and saying BS that a semi auto version (AR-15) can fire the rounds per minute.
And some are just not too smart. Think of 'Chief Moose'.
OMAHA, Neb. Omaha police Friday released three still images captured from surveillance footage inside a shopping mall department store before Robert Hawkins' bloody rampage began. One shows the 19-year-old walking through Von Maur's front entrance with an apparent bulge under his sweat shirt. It was an AK-47 assault rifle, police said.