When guns are outlawed only outlaws will have guns.
In the US's case, yes, that's true.
Certain types of gun restrictions are only effective when it's preventative and wouldn't work in a prescriptive sense.
Meaning, the types of things that may have some efficacy pre-proliferation won't work post-proliferation after there's already hundreds of millions of them already in circulation, and they're not going to magically stop functioning anytime soon.
And certain measures like that tend to only work if there's a certain level of "social buy-in" to the premise.
That's why the measures that worked in places like Australia and the UK would be abysmal failures if tried in the US.
To kind of put things in perspective. The UK already had a registration process in place prior to their restrictions (much of the US does not)
At the end of 1995 (the year right before they really clamped down on gun ownership), the UK's situation was:
At the end of 1995, 174,000 firearm certificates and 723,000 shotgun certificates on file
(their population was 58 million people at that time)
Or, in other terms, they had 1 gun for every 65 people. Obviously that's not where the US is at.
They also had a social buy-in, as when their population is so disinterested in guns that fewer than 1 in every 50 even bother owning one (and the ones who do mainly do it for hunting/sport reasons), there's not going to be a tremendous amount of pushback on such kinds of legislation.
I've used this comparison before, but the comparison to alcohol prohibition is valid.
There are middle eastern countries that have outright prohibition on alcohol that has lasted decades, and there's not much social turmoil surrounding the topic. Reason for that being, 99% of the population was already abstaining from it and had no desire to drink it anyway for religious and cultural reasons. It would play out very differently if a prohibition on wine was being proposed in a place like France or Italy where there's a deep cultural attachment to wine. (and, in fact, did play out very differently in the US when we tried to enact prohibitions on it)
With where the US is at today, trying to enact British-style gun control laws would work about as well if as if we tried to enact stricter drunk-driving laws, right after we gave everyone a free 30 year supply of vodka.