Hopefully you're never in the situation where it's essential - but if you were in Colorado before it became Kolorado, you could have been looking at: "If I don't buy MP5 magazines now, any future MP5 I buy will have to use 15-round magazines if they even exist" and then swap MP5 for any gun you might ever want...

For rare guns/magazines it's even worse "I may not be able to find one of these.... ever" Go find a Wildey magazine for instance - and that's not even mentioning the price.