A sword could be hidden lots of ways..
Umbrella handle/cane shaft are two fashionable and plausable items.
Next is sporting eqiupment like Hockey sticks, baseball bats, and the like.
As for a full on Assault rifle?
Maybe a guitar case ( for that "El Maricachi" feel) or similiar item.
If you change out your AR for a carbine, you might get it down to a briefcase size (folding stock, no barrel mounts, short barrel mod), and still be rapidly deployable. Of course, an Assault Rifle is not meant to be hidden, it's all about that huge unstated statement: "I have an Assault rifle, what are you going to do about it?"
If you are looking at trying to be sneaky, think SMG for a punch. Much easier to hide, burst fire, high ammo count...