Like others have said, different designs and frames require different mags. With that said, there are some firearms that take the same exact mags. I own lots of Glock, and pro mag stunt doubles, that will also work in things like Kriss Vector, BRO AR pistol, and Keltec sub2k. So, if you were building collection around mags, which is little silly, maybe, but you could start with those four different firearms that could all use same mags, depending on caliber (I’m sure there are more firearms I didn’t list too). I have all of those and it is handy utilize same mags for them.