CAUTION: BIG POST AHEAD!
Good question.
If it had reduced the number of deaths by homicide on an average year by say.... 5-10% Yeah that's probably enough to call it a success.
It would have to be statistically significant enough to say that it has had a positive impact and has definitely saved lives to a statistically measurable amount. That would be enough for me to call it a success.
Other people might want it to save a higher percentage to justify the loss of freedom, and I might well have argued that at the time. But today I'd argue any measurable saving of lives in the actual homicide rates and such would constitute success.
Arguments can be made that reducing the number of mass shootings should be considered a success, and I'll admit to finding that argument pretty persuasive. What makes me want to dismiss mass shootings is they make up such a tiny number of people statistically speaking.
It's still tragic, it's still peoples lives we're talking about. But there are costs for having such laws in the first place. While in Australia there wasn't much if any kind of armed self defense culture anyway, to the point that I didn't put much consideration into the number of people saved by having a firearm to defend themselves, I quickly determined that to also be below a statistically significant quantity as to be worth including.
But there is also weight to the argument of personal freedoms, to the ability of people to feel safer, as costs of gun control that need to be considered along with how many people it might save.
Again, I appreciate that Australia didn't have an armed self defense culture or mindset to start with, but this is something you'd definitely have to consider in some countries.
And, getting more to the point. Say there's no statistically significant impact on homicides, which I believe to be the case, lets just take the mass shootings.
By reducing mass shootings, how many people do you save? Maybe a few dozen over a 5 year period. Once you start considering factors that are that... I hate to say this but, adhering to cold numbers here, that insignificant (again I don't want to come off heartless here, it's just that in a country of 25 million, 50 deaths in 5 years is statistically insignificant). Once you do that, You have to also start weighing in things like the impact on peoples lives in other ways. There are a lot of people for whom shooting is a sport, this significantly reduces and hinders their recreational activity.
Additionally, and this... this really gets to the heart of my quasi-pro gun beliefs. One major reason I hold the beliefs I do. I mean originally I would say if I'm being honest that I was brainwashed into them. I grew up around some very right wing progun type people and was taught from a fairly young age that guns are good anti-gun people are idiots liberals suck etc etc. Over time as I got older, and especially after moving to Australia and seriously questioning my beliefs, I stopped holding a lot of the right wing beliefs I once held. But one major part of my beliefs I've never stopped holding is my belief that people, and especially women, have a right to defend themselves.
I've never liked that because a bunch of young men, and it's almost exclusively young men, misuse guns and kill a bunch of people, that women are prevented from having a gun with which to defend themselves.
To many times I've seen or heard of women being stalked by dangerous men, being let down by useless police and ineffective restraining orders, and having to live in fear.
A gun provides another option, it provides a way to take back a degree of control and defend oneself and not be dependent on police response time.
These are also people impacted by these kinds of laws. Now I would be tempted to say that it's perhaps still worthwhile if you can save thousands or even hundreds of lives a year by introducing gun control. But a dozen or so over 5 years? I'm not so certain.
And it doesn't HAVE to be this kind of gun control or no gun control!. There's no good reason we shouldn't try gun control that strongly clamps down on who can have guns but still allows women (and men) to have weapons to defend themselves with in these sorts of situations. As well as letting gun enthusiasts to continue their recreationally activity. While still saving lives!
That is why I don't like the Australian flavor of gun control. And why I think there are better ways, and why I don't consider it much of a success. If you're read this far though I wanna say I appreciate it.
One last thing, you'll note that among this 'pro-gun' argument I don't include society defending itself against a tyrannical state. And it's because like you I consider that argument a joke.