In order to 'get' 'assault rifle:
Citizens, save up the money, buy what you want.
Bad guys, steal or dope deal or pimp to get some money, steal or have a 'Straw Man' buy what you want.
Big City Cops, go to your Administrator and convince him you need an assault rifle, may or may not happen. Chief requests funds to buy assault rifles from council, may or may not happen. Test several brands and makes, get the brand/make the head of testing team will soon be working for. Purchase assault rifles. Administrator will now issue these to a limited number of Officers who will go through hundreds of hours of training. When needed, an Officer with 'The Assault Rifle' will drive across town to the location hopefully getting there before anything happens.
Small town Rural Deputies, use credit card buy whats on sell OR if there is one that has been seized talk the Chief into letting you carry that one, maybe.