Except that the whole point was they should NOT have been taken by surprise. There was plenty of time to realize something was wrong and automatically assume that someone could come to the armory. If they were truly trained, then it would have been drilled into them that when you are on guard duty, you always assume a threat until proven otherwise.
They should have had their guns out and been suitably apart, taking whatever cover they could. The moment Simeon appeared and shot, the second guard should have been returning fire. Simeon should never have reached the gate room, nor should he have even been able to get off the ship.
The better way to handle this would have been for Simeon to take a hostage before all of this, possibly Ginn/Perry, and use that to get to the gate room. Our military guys wouldn't have looked like idiots, then.