Big magazines for the right guns only!
Hey choomb, you have a curved mag in that pistol!

No, no, it's not that simple. My idea would have different "tiers" of magazines. Is there a 9mm pistol and rifle? Yes! Is there a 10mm pistol and smudge? Yes! They can both be loaded with normal or extended magazines, which is a little...odd. Don't even get me started on drum mag pistols...

A pistol should have the following:

A 'standard' magazine of 10+1 (maybe even 7+1 on a .45?)

An 'extended' magazine of 15+1.

Rifles and smudges should have a new type of magazine: the 'curved' magazine. This would replace the standard extended magazine, plus option drum magazine if you want even bigger capacity.

I cannot stress enough that by making rifles and smudges the only weapons that can fit 30 rounds in them (which is very realistic), we'll make players look towards them as a skill more.

I see a lot of guns in my day job and 30 round magazines in pistols are common. Folks on the street call them "poles". Drum magazines are rare in pistols but I still see a pistol with a drum magazine maybe once every 2 months or so.

Most people who are regularly using guns to shoot other people are not very well trained with them, since they can't go to a gun range, so they rely on (often ridiculous-looking) extended magazines to put more bullets downwind. Folks on the street very rarely are carrying reloads like someone in the police or military might so once they are out they are out.

My limited understanding is that one of the benefits of pistols versus other types of firearms is that they can be concealed.

Limiting their concealability when equipped with a high capacity magazine might be enough to entice people to go the SMG route instead.

Personally I think that SMGs and rifles are not more widely used is due to perceived availability and cost issues.