New Zealand didn't pass a ban and also had no mass shootings (which, by the way, Australia has had since, definitions being tricky things) and the US didn't pass a ban and has also seen drops in people murdered, including with guns.

You can't give the laws credit for something that was decreasing prior to the law and continued to decrease at effectively the same rate after.

Further, "death by guns" is a nonsense statistic. It is the total change, or lack thereof, in overall homicide rate trends that matters, not any given means. Criminal death has moral value, inanimate objects do not.