These guys sound pretty incompetent.
9mm.
I'd like to know how many people who aren't farmers, hunters, or vets know how to put down an animal with one shot. I know not because of my law enforcement work but because I'm a sheep farmer ... and even the prescribed method doesn't always work the first time out.
Even then, most people who aren't involved with death up close and personal don't understand that even a well-placed head shot doesn't always lead to instant death. You may have taken care of any lingering consciousness, but the body systems don't just stop like they hit a wall.
Having said that, running over with a cruiser was NOT the best idea of the week. (I think they carry S&W .40 in Arlington, but I could be wrong -- however a well-placed nine would do. Heck, a well-placed .22 would do it.)