You could, for example, make it illegal to sell/own guns under 30.

When do people become adults? I think if we're doing shit like this we also have to raise the age of adulthood to (in this case 30). Meaning no military until 30, no smoking/vaping/drinking until 30, on your parent's insurance, they can't kick you out until then, can't be tried as an adult, age of consent, etc, and frankly we likely shouldn't trust kids with other deadly instruments such as cars until then either, they're clearly causing problems with those too considering the numbers on car accidents.

Frankly there is some scientific basis for all this, being that our brains don't stop developing until 25, but this half-adult stage where all of the negatives but none of the benefits apply is ridiculous to me. Fwiw I have this exact same argument about raising the smoking age, etc, it's not just guns, but talk of raising the age makes them relevant to that opinion for me. I'm not even necesarily against it, but for me personally to be on board it has to be bigger than "no guns til 30 just because." Besides, people 18-29, especially women, have a very good reason to have tools with which to defend themselves, and I personally think it'd be a shame to deny say a woman who is escaping an abusive ex those tools, especially considering often women date slightly older men meaning he may be able to get them while she can't, if she's 25ish and he's 30+.

You could ban magazines

Not really. You could ban detachable magazines, which is 99% of guns made after 1910, but that won't fly here because duh, you could ban detachable magazines over 10rnds, but that is entirely ineffective as it's defeated by "carrying another magazine or two," and reloading takes less than 2sec literally (way less if you train it which you can do in your room while watching TV.) That also includes almost every 9mm pistol in the country btw, which mostly have 14-17rnds standard, magazines aren't only in AKs and ARs.

You could limit the caliber of bullets sold.

I mean, in theory maybe that is legally possible? Maybe? But even still the venn diagram of "the deadliest calibre" and "the smallest calibre" is actually just a circle containing the word ".22lr" This is mostly due to quantity (availability/price), of course, but at any rate it illustrates that limiting calibre wouldn't do much since even the smallest calibre commonly available can kill with ease. On top of that the .223 rnd commonly used by mass shooters for example is .003" bigger in diameter than a .22lr, about 25gr heavier of a bullet, but with more powder making it carry about as much kinetic energy as a hot .357 rnd, they don't usw "high powered" rounds like the .50bmg or .338 lapua already, .223 is one of the weakest rifle rounds commonly used for self defense or hunting coyotes.

You could make gun manufacturers civilly liable for the advertisements they put out.

Where are you guys seeing gun ads, like, in Guns & Ammo magazine or something? I don't think I've ever seen one but I hear about them with the whole Daniel Defense scandal. At any rate, they already would be to a degree, like Juul was, that would apply to guns too if it is proven to target kids or something. But there would be a court case to determine culpability which they could win, that's just the way our system works really. Seems easy enough to just not buy those magazines for your kids or whatever. Do they even still have print magazines?

I'm not opposed to something that would actually work and couldn't be abused by some racist sheriff or governor to deny guns to POC et al. But most of the proposed legislation I've seen falls short of one or both of those (personal) requirements. Most things will be used to further burden marginalized populations in overpoliced neighborhoods just like drug prohibition is, it won't affect the people in gated communities at all.