When Murdoch examines the spent cartridge case under a magnifying glass, the headstamp "WCC 87" is clearly shown. This indicates it was manufactured in 1987.
Earlier in the episode it was established that Gillies wore a mask of Dr Ogden and killed Darcy by pretending to be Dr Ogden to fool witnesses but later on it is also established that he set up an automatic device that can shoot bullets while he was busy miles away getting rid of Dr Ogden's alibi. He can't be in 2 places killing 2 different people at the same time.