This is such a US thing. A rifle for hunting and self-defense, sure. Piles of fully automatic AK-47s that chew through 600 rounds per minute, not much.

And the old trope about ammo becoming currency, has this ever actually been a thing anywhere?

There's an interesting paper from WW2 about the shadow economy of a prisoner of war camp, fuelled by imports of Red Cross humanitarian parcels. Cigarettes became currency even among non-smokers. I'm not sure what the equivalent goods would be now.

I was being somewhat sarcastic and hyperbolic. Fully automatic weapons aren’t even legally sold.