Tbf

go out after a drunken fight and decide to shoot someone,

I highly doubt a drunken and bloody person is completing a sale even if it’s legal (and I don’t think it is). If they can wait until they sober up, take a shower, and then still want to go to the store the next day, three more days isn’t likely to assuage their anger, and even if it does then the next time the situation arises the gun is in their posession already.

or an isolated depressed individual to say this is the night to die,

Guns are the most frequent choice for men, sure, but not even for women (that would be intentional poisoning last I checked). As such any sufficiently motivated man can just go the woman’s route which is technically even more convenient since everyone has a bottle of tylenol on hand. Or, guns also aren’t even the most effective, and I hesitate to say what is lest someone depressed see it and they decide it’s a good idea, but suffice it to say it is located within 5mi of your house right now (in most countries) and it isn’t guns nor is it something that can be remedied. Furthermore 72hr isn’t sufficient time for such an individual to seek professional help, may not even be enough time to get them to their first appointment, any such individual will have the same scenario as above, in which while they may be prevented from suicide “tomorrow,” once the purchase completes they’ll have it for next time. In fact, CA found that risk of suicide by firearm is highest in the week after the purchase completes and stays high for months after.

or to keep another of the far too frequent school shootings from happening.

This is probably the least effective, these kids plan these shootings for months. They aren’t saying “mommy, can you drive through Ammu-Nation on the way to school this morning? I’ve been good all week!” and picking up their arsenal. There more than likely is at least three days from point of purchase to time of crime just due to logistics in most shootings (counting purchased-by-shooter guns, stolen-from-daddy of course is longer than three days from purchase to crime, wouldn’t be fair to count it as it would artificially inflate the numbers in my argument’s favor.)

In fact speaking of “time to crime,” as the ATF calls the time from purchase until the gun shows up at a crime scene, the average time to crime of guns in the US is eleven years. No state has an eleven year waiting period.

But think for a second about a woman escaping her abusive husband and taking the kids to her mom’s. It may be beneficial if she knows her abusive husband is armed for her to buy a gun of her own as he’s clearly dangerous (and taking one of “his” could get her imprisoned and give him the kids, not an option). If she can’t get it for three days and he knows that, that could give the armed abusive husband just one more advantage at wholesale family annihilation (ala The Haight Family) by denying the victim the tools (and therefore ability) to defend themselves from harm. Unfortunately, if she calls the cops before he gets there, even if she has a reasonable suspicion he’s on the way, they will not do anything “until a crime has been committed” and therefore she’s unfortunately left in a position where it may be necessary to do so.

Not only are artificial waiting periods ineffective for their stated goals, the hindrance could also actively prevent those that do have a sudden need to acquire a firearm from protecting themselves from harm.