He was shooting a revolver. The ammunition in a revolver isn't hidden in an internal magazine; the business ends of the cartridges are plainly visible from the front without even flipping the cylinder out. Wouldn't a simple final external inspection have revealed that were bullets loaded rather than the crimped end of a blank cartridge?

they apparently use ‘dummy rounds’ that ‘look real’ when scenes call for the gun to be viewed where the audience can see the cartridges i n the cylinder- so they want them to ‘look as real as possible’ - but still- IF they used those types- the actor should have checked to make darn sure they were dummy rounds and not the real deal- He’s the one with the weapon in his hands now, an has the final responsibility to make darn sure it’s safe-
According to experts speaking about movie set scenes— the guns should not even be pointed directly at people- angles should be used to give the illusion that they are pointed directly at-

“flipping the cylinder out”
Cylinders in vintage single-actions Don’t flip out.