[JshWright]

What she meant (I think it's safe to assume) was something along the lines of...

  While it's obvious to you and I standing here examining it that
  it's a prop gun, if it were being wielded in a threatening
  manner, it would be far less obvious, and it's reasonable to
  assume that it could be perceived as a real threat.

Given the fact that there are several models of gun that small that are quite capable of killing you, that seems like a pretty reasonable argument to me.

[bullseye]

I think you give the agent too much credit, but perhaps that was the thought process. I'll concede that point.

However, I still think it was a dumb argument. Note that the closest small gun you linked with any chance at being lethal was 4-5 inches long. A gun half that size, which someone would have to basically hold with their thumb and forefinger, cannot possibly be menacing in any circumstance unless you are a Smurf.