The revolver, for those interested, is a Nagant model 1895 and unlike any other revolver, seals the cylinder to the barrel just before firing so that it can be effectively suppressed, hence it being drawn with a sound suppressor.

In other revolvers, there is always a gap between the barrel and cylinder so that the cylinder is free to rotate during normal operation. Through this gap propellant gasses escape, creating a second blastwave other than the usual muzzle flash which is equally audible and in many cases equally visible, if not more so.