Pure speculation on my part, but when I first read the round passed through the lady cinematographer and then hit the guy, assuming they were going for western authenticity, I guessed it was most likely a .45 Colt with a standard 255 grain lead round nose.
The pistol that I saw pictured looked like an 1851 Navy Colt. If it was a cartridge conversion, it may have been chambered in .38 Special.