No one paid attention to the gunshots that echoed through the convention center. They were real enough, and so were the screams that accompanied them, in the sense that they were recordings of real people who, like guest stars on Law and Order , reenacted scenarios that had clearly been plucked from the headlines: a kidnapping, a mass shooting in a church, a riot on a city street. The auditory terror punctured the otherwise banal din of an industry conference. This terror was, in fact, one of the products on display: the V-300 S-Screen Simulator, developed by VirTra, one of the 193 vendors at the annual Border Security Expo in Phoenix, Arizona. For more than a decade, vendors and government representatives have mingled at the Border Security Expo, an annual trade show at which the former hawk their goods to the latter, promising that this camera or that sensor are the key to locking down the border once and for all.…