IF the victim believes that showing the gun is an indication that non compliance will meet with deadly force (reasonable in this circumstance) not only is the showing an assault, it is an element of an armed robbery. Read again, an assaut occurs in the MIND of the victim, NEVER the intent of the BG.
If you show your gun to someone, kidding around and they believe you threatened them, YOU are guilty of ADW, not merely "flashing" or failure to conceal the weapon. In the other person(s) mind, they have been the victim of an assault by you!
The law is pretty universally clear on the elements that constitute assault and this is not a new concept. Trust me on this one!
Another way of looking at it, Robbery with a gun consists of an armed assault and a larceny. The ADW is considered a "Lesser included Offense". Sometimes in a plea agreement, a robber is allowed to plead to the larceny part to avoid the much stiffer penalty for the assault portion of the crime IF no one is actually injured. However, anytime a weapon is shown with a demand for something you have, you have been assaulted in the eyes of the law. Could you say there was no assault if, instead of flashing and showing you the gun he had held it pointed AWAY FROM YOU? No and the same legal principle applies, even though the gun was not pointed directly at you, you understood that showing it meant he might use it.