There is a vast difference between the Cowboy Action Shooters firing off their horses they've trained to ignore gunfire -- blanks -- and the old timers and cowboys who actually rode horses for a living and might actually have had need only rarely to fire from their horses' backs.
The CAS members have fired literally
dozens of thousands of rounds each, training themselves and their horses for "action shooting." Fun and games. Back in the old days, very very few cowboys, sheriffs, ranchers & farmers, even soldiers, could expend the money to practice any acceptable accuracy required to shoot from their horses' backs. They just could not afford it. When a Colt's six shooter cost near the equivalent of a month's pay, and a box of cartridges cost upwards of $1.00, money was too dear to go blasting it away at tin cans and empty whiskey bottles.
As for what you have seen for nearly 100 years in the movies ... boys and girls, do not believe any of it. All that stuff up on the silver screen or the teevee tube, is there for one reason and one reason only: entertainment. Authenticity has virtually no place in western flicks.
Believe me. I know. 
Just relax, have some popcorn, a Coke, and finish off with a Snickers bar and enjoy the flick.
L.W.
