I think Catherine said that about the bullets not working out of past experiences.
The security guard idea was done out of poor writing I believe. They didn't consider we, as fans, would realize in this situation that those guards would be useless. It seems so, anyway. Because Tess is a smart character. It just made her look dumb.
Any superhuman could snap their necks faster than they can fire a bullet probably.
One of my many vents about the show
