What does Travis mean with his comment, "that's stretching luck" in the short story, "A Sound of Thunder"?

As he travels in the Time Machine of Time Safari, Inc. in which he has contracted for the opportunity to kill a Tyrannosaurus Rex, Eckels asks the Safari Leader named Travis if it is possible to bring down the dinosaur "cold" with the guns that they have with them. Travis replies that it is possible

"[I]f you hit them right by shooting them in their two brains, one of which is in the head and another a good ways down the spinal column."

But, he tells Eckels that the guides try not to shoot in these places on the dinosaur because doing so is really taking a great chance:

“If you hit them right,...Some dinosaurs have two brains, one in the head, another far down the spinal column. We stay away from those. That’s stretching luck. Put your first two shots into the eyes, if you can, blind them, and go back into the brain."

With his statement that being able to shoot the giant in both the head and the base of the tail is "stretching luck," Travis implies that the distances between these two areas necessitate the hunter's being able redirect his aim and be accurate and quick, as well. In the time that it takes to fire the shots, the dinosaur might reach his attacker, and if the shots are not fatal, the Tyrannosaurus can, then, kill the hunter. But, if the hunter shoots it in the eyes, which are close to each other so very little realignment of the weapon is needed, the hunter can then blind the creature so that it does not know where the man stands. With the giant blinded, the hunter has the opportunity, therefore, to fire his gun more times and successfully kill it and not be harmed himself.