Ehhh…. Shotguns can shoot buckshot or rifled slugs.
Just a guess here, but I suspect there are a lot of bird hunters in the UK. I suspect that that is why shotguns are legal. It has nothing to do with how dangerous they might be.
I doubt very seriously that either buckshot or slugs can travel as far as center fire rounds. I’m open to the fact that I might be wrong so I’d love to see a cite.
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I haven’t noticed anyone answering this question, so I’ll take a stab at it. The magazine is the part of the weapon that holds the cartridges prior to loading them into the receiver which is where the cartridge is fired from. Most weapons these days (not counting revolvers) have detachable box magazines, inserted either in the handle (as in the M1911A1) or ahead of the trigger (as in most rifles and the C96 Mauser). You can load the magazine ahead of time and carry several if you think you need that much ammunition readily available. Other weapons have an integrated magazine, one you can’t take out and reload handily. The 44-40 cowboy Winchester carbine had a tubular magazine under the barrel. You fed it cartridges by pushing them through a gate on the side of the magazine. Others have a magazine well, again just in front of the trigger.
Clips, OTOH are more-or-less disposable metal bits that keep the cartridges together until you can get them stuffed into the magazine. Some go into the magazine, others do not. The M1 Garand is of the former type, holding eight cartridges in an en bloc clip. Clip and all, they are pushed into the magazine. When the clip is emptied, it is ejected from the magazine so you can push in another full one. If you saw Saving Private Ryan you’ll recall the distinctive ‘ching’ of a Garand clip as it flew from an emptied rifle.
Stripper clips are much less substantial than the en bloc clip and do not follow the cartridges into the magazine. You push 'em in and discard the clip. Here is the above-mentioned C96 Mauser with ten cartridges in a stripper clip poised to load its integrated magazine.
This is all formal gunese, of course. As Lumpy mourned upthread, even people who should Know Better will refer to a magazine as a clip.
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The ‘lowly’ .22 as you called it is not a centerfire round. Though under perfect conditions, it could probably go a mile. Unless you are talking about the .223 or swift. The .22 in common venacular is the rim fire .22 that most of us grew up with.
Understand that the warning on a box of .22s or any ammo, is how far it could be shot under perfect condintions. And shooting it like a morter. Not a rifle.
If it’s falling out of the sky, I would much rather be hit with bird shot. On the other hand, I really would NOT like to be hit by a slug or 00.
12 gage 00 buckshot is much more likely to do damage than a .22. Though I doubt it could go a mile. It doesn’t need to. One shot from a 12 ga with Buck Shot throws out 8 to 12 .30 caliber balls at over 1000 fps. A bit less fps than one 40gr of a standard long rifle .22. YMMV 
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It is true that lighter bullets have higher muzzle velocities (less to push out) and it is also true that they have more energy (E=MC^2), but there is another aspect to consider: overpenetration. From a purely objective standpoint the smaller bullet is better, but the numbers don’t account for what may be behind the target.
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If your ballistic charts use E=MC^2 for energy, you have to put away the ‘special weapons’ book. 
Kinetic energy is 1/2(MV^2), and momentum (the important one here) is P=mv.
I personally (rifle shooter here) like light, fast bullets, rather than heavy slow ones. They also tend to have less kick. On my .30-30, I use 150gr bullets, rather than the bigger 170gr bullets… but it’s more preference than real effectiveness with the game I hunt.