Swords are popular.

You might not have realized that, but it's true.

Many theories exist as to why this is so. My theory is that it's because swinging sticks around is fun, but they don't look cool. Swords do look cool. They're the natural upgrade, and everyone wants one.

There's also the simple prestige of sword ownership. You can just walk around trees and find a random stick, but swords? I've walked for hours in my local park looking for a random sword, but do you know how many I've found?

Two, and they weren't even that good.

Anyway, my point is that while I don't think this exact concept has been done, I'm certain many like it have been. There's no way swords that create explosions on impact hasn't been done. I can't think of an example, but it definitely exists. I don't even need to check.

As for people forgetting how guns work, or, more likely, forgetting how to make gunpowder? Also probably done, but again, I don't personally know of an example.