And there we have it: the key to understanding this debate.

The argument is not deterministic, either way. But even if Mercy is right that high-capacity weapons don't necessarily make shooters less efficient, what flags is saying is rooted in reality---pedagogically speaking, it may be better to start shooters out with lower capacity. There's more than one way to do this, though; flags has mentioned not loading a 9mm magazine all the way when shooting more than one target. One needn't have a revolver to force themselves to do more with less.
I've elsewhere said it probably makes more economic sense when buying a first rifle for a kid to get a bolt-action they can use for several years (if not their whole life) and only load a single cartridge, than to buy a single-shot rifle, only to replace it years later.

Of course, people have to hold themselves to this, or be held to it by someone else. But if they cannot resist the temptation to load the magazine all the way at the range, or load their magazine all the way in the woods, it's probably a poor bet that they'll overcome the temptation to buy anything other than a revolver or single-shot in the store.