re: What rifle/pistol round got most popular the fastest?

My argument says that I would say demand but I truly believe that is the case. I know that remington jumped in head first with materials and ammo. Additionally, any ammo manufacturer for say 06 is making 300 Blackout ammo too. It was hard to find before December and now it's impossible. Manufacturers are even claiming that they are switching from 308, 7.62, and 5.56 to start making 300 because they can charge a bit more and the others are catching up.

The biggest hold up is that there is no new 300 brass. It's all 5.56 that's been cut and sized. Hard to convince some manufacturers to add the extra step when 5.56 is so easy.

So in the end, I guess it's a bit of both but the round isn't dying, it's actually too successful for its own good.