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.