Replacements or advancements
Gun magazines are designed for the caliber or manufacturer that they are meant to. Some rifle or pistol magazines can work in different firearms, for instance, the Beretta Storm 9mm can take Beretta 92 magazines and you may be able to find capacities for your guns that pistol magazines aren’t usually for. Putting anything more than a 10rd or 15rd magazine in a 9mm Pistol can be awkward, but put a 20rd mag that works in your Beretta 92 into a Beretta CX4 storm is an advancemnet.
Any AR15 or M16 Operator will know that what the military uses for these firearms is lightweight and funded by Uncle Sam, so nobody really cares about fixing springs or followers once they get old, they usually get tossed. MagPul Pmags are hands down the most durable Carbine magazine on the market and have greatly increased the reliability of many direct impingement firearms. There are several gun magazines manufacturers that are 2nd rate, but sometimes a good deal on magazines for training or range use can save you money. If you’ve got good magazines like Chip McCormick or Wilsons Combat mags for your 1911, you may want to use those for your carry guns, but not want to wear them out when you’re testing your handloads or just sending lead downrange at your local gun club.