We probably don't know what hard times are.
What was the last big thing for firearms? Not so easy to say. But chances are, it won't be revolutionary. It will be evolutionary. You probably won't even realize it.
Here's one example. Optics.
There's nothing revolutionary about optical sights. They've been around since before my grandfather was born, which was 1876, I think. But look at photos of soldiers. American, British, Australian. They all have rifles these days with optical sights. That is, the infantry does. That would have been considered something else when I was going through basic. And judging from on-line catalogs of hunting rifles, hunters are expected to use them, too. It isn't revolutionary but it isn't L.L. Bean going to the woods with his .25 Remington autoloader anymore.
I also don't see non-metallic cartridges going away very soon either. In fact, one could say that maybe metallic cartridges may have been the last big thing in small arms, just before smokeless powder, high velocity cartridges, and automatic principles, to mention just a few. I doubt metal prices even enter into the picture and anyway, steel cases have been successfully used for a long time. However, tank ammunition is mostly combustable, so possibilities abound.