Just adding on to this, I found this study of Australia’s firearm reform.

https://www.semanticscholar.org/paper/Fatal-Firearm-Incidents-Before-and-After-1996-Chapman-Stewart/9146a5543654d64227c688abc8a1a821f665039c

An analysis of firearm deaths between 1979 and 2013 showed that 13 mass shootings (homicides in which at least 5 persons died, not including the perpetrator) took place in the 18 years preceding and including the Port Arthur massacre; none has occurred in the 22 years since

Before 1996, approximately 3 mass shootings took place every 4 years. Had they continued at this rate, approximately 16 incidents (SD, 4) would have been expected since then by February 2018.

Now this could easily be attributed to the other changes made. The study was not looking specifically at a register but at all the changes made:

Provisions included uniform gun registration, repudiation of self-defense as a legitimate reason to hold a firearm licence, locked storage, a ban on private gun sales and civilian ownership of semiautomatic rifles and pump-action shotguns, and standardized penalties

I would welcome anyone that can find a study on gun registration independent of other changes, this is the closest I have found: https://www150.statcan.gc.ca/n1/pub/85-002-x/2022001/article/00009-eng.pdf

It shows after Canada ended registration for rifles and similar in 2012 gun crime rates slowly rose after slowly falling up to that point, but I am not convinced this is related (and the document makes no connection as far as I can tell). Canada still registers hand guns and similar, which are the main guns used in crime, so this is in no way definitive one way or the other.