"Even in the contest between man and steer, the issue is not certain."

why don't you like the movie?

It’s a good movie, it does a great job of showing murder as absolutely horrifying.

4 stars for those and a 5th one for the stains on the muzzle.

He explains that the big chamber in front of the barrel prevents/blocks the spray that you'd get if the barrel was right up to the animal. This also has a design to get the right angle to the bullet goes right into the brain...

I mean he explains this in the video.

Well For Some Reason When You Said It It Made More Sense!
So, thank you for that.

Also, inferring from what he says in the video about why that angle was chosen (to target a specific region of the brain), if your aim is off by just a couple centimeters the horse is probably not going to die quickly or humanely, and it would probably be hard to be that accurate on a battlefield with a struggling, wounded animal.

as jfcaron_ca and OZ said, preventing back spray and missed shots. During war time, one of these probably got used dozens of times during any given day, so practical expediency was a big concern when it was designed.