I'll bet, due to budget constraints, a safety officer was not present on the set. A safety test is supposed to be done before handing the gun to an actor. The safety officer aims the gun loaded with the blank at a sheet of paper placed at a distance from where the "victim" will be. If any ejected gunpowder reaches the paper, different blanks with less gunpowder are tried until the right one is found.
An untrained prop person probably loaded the gun with live rounds.