The only hunting regulations regarding guns that I know of, and that are worth mentioning here, is that you aren't allowed to take a shotgun or a rimfire hunting for bigger game on our government land. That is sensible when it comes to protecting our native birds etc.... and to help prevent irresponsible hunters from injuring animals with inadequate fire power. Some folks might not be so careful with bullet placement and responsible distances to shoot. On private land you can pretty much do what you want.
I like to use a .22 near built-up areas for safety and noise reasons. I also like to use a .22 in other places for convenience and noise reasons. Nowadays I often have a supressor fitted to the .22 I'm using. I simply don't like to let people know what I'm up to, and I don't enjoy loud bangs.
Having hunted with a primitive bow and arrows, I realize how much better a scoped, suppressed .22 is. And I've only just started using a scope. When the rifle is properly sighted in, it is amazing that those little bullets generally hit where the crosshairs are resting. I used to think that scopes were easily misaligned, a nuisance to carry, and maybe a bit sissy. Nowadays I'm convinced that it is an important part of my .22 meat-getting system. I am a reasonably good shot with any sights, but a scope almost makes me superhuman by comparison.
I still use a .22 with open sights occasionally, but I would not take some shots on larger animals if I was uncertain about hitting the right spot.
To me using a .22 responsibly involves a reasonably good level of marksmanship, knowing the limits of the gun and the shooter, knowledge of the anatomy of the animal, and the self-control that allows you to not take shots that you aren't certain will kill.
Head shots certainly drop an animal, but folks need to be fairly sure of the location of the actual tiny brainbox that you need to hit. And on some animals it can be well protected by bone..... especially when shooting at some angles.
Head shots on hogs can be particularly dicey. And a straight shoulder shot can be stupid on a larger animal because of the thickness of the shield. I've found that with smaller hogs an angling shot behind the shoulder will reach the heart-lung area and bring the pig down quickly. I would be reluctant to shoot at a bigger hog with a .22 unless I was very close and could put a bullet in at the right angle.
The black and white pic of me shooting at a hog was taken at a time when I anticipated maybe shooting a goat. I was also only about 18 years old. It took several shots to finish the job... I'm glad I didn't have the single shot Gecado Plinker .22 I often used in those days. The Anschutz's box magazine saved me.
If I am seriously after a pig or a deer nowadays I generally use an old military Lee-Enfield that my father converted to shoot the 7.62 x 39 cartridge. It is superb for the job provided that I don't want to take long shots..... although I was particularly pleased a while back to take a stag with the little cartridge using open sights at a distance of maybe 140 yards.
So I don't recommend a .22 for larger animals, but used carefully this little cartridge, shot from a reasonably long barrel, will suffice as a reliable tool for hunting.