Actually there is no such thing as an "Assault Rifle".. It's a term made up by politicians and the media.

There are bolt action rifles, Semi-Automatic rifles and Automatic rifles.. I guess there are others such as muzzle loaders but those are the main categories.

Even though Assault rifle has become an excepted term, it still only applies to military automatic rifles, not civilian semi-automatic rifles. Unless of course you are a politician or the media trying to scare the public.

As far as an AR-15 type ammo going through a vest? Very easily. Rifle rounds, though often have a smaller diameter, they travel at a much higher velocity than a pistol round. Police vests are designed to defend against pistol ammo. If you throw in steel tipped (or as the media likes to refer to as Armor Piercing) it makes a standard vest seem like butter.