It does look like the "victim" is in costume. The coat sleeves seem too long. What is up with his eyes? The mustache is odd looking. Also, the stance seems to be sustainable for a still photograph.

The shooter is not in recoil and there is no puff of smoke, even if it is "smokeless" powder there should be a hint of it.

So I vote that it is a staged photo. Perhaps as a visual advertisement of a theatrical play and displayed with other photos.