Quoted:
I know lots of stuff is scarce currently but mags aren't.
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Mags aren't exactly scarce, but they are selling out quickly and consistently when they are in stock. Manufacturers are producing them as quickly as they can, but I frequently see many types of mags sell out, get replenished a few weeks later, and then sell out again at the major online retailers. I'd say that the rate they are being produced is keeping up with demand, but things still sell out temporarily.
It's possible Lancer has shifted more manufacturing capacity to 5.56 mags, since those are always the most popular, but I've seen no indication that they've discontinued the 300 BLK line. I'm sure you have already, but I'd stay signed up for notifications from Primary Arms, the Mag Shack, GunMagWarehouse, and others.