Joe can flesh it out more, but his thinking is that you've got two guys somewhere up high with line-of-sight. One fires off a round that doesn't even try to strike Kirk, it's just fired overhead somewhere, or just a blank bullet, and the other simultaneously detonates the booby-trapped mic/battery-pack on/under his shirt. Robinson is over there at another location, the Losee building, where he's meant to be seen, thus drawing attention to him. The gunshot provides, well, the loud gunshot sound, but the hi-tech device guarantees his death (and, perhaps, the second, odd, slightly-off-time-wise 'thud' sound that's heard).