That's not how backing works...

I know that. But if I'm given the chance to change the way it works, why limit myself? Choosing a random commodity is boring and doesn't solve the underlying problem.

you know the government's just going to produce more cheaper and low quality weapons especially for destroying if that's how the value of the dollar works.

Hence the destruction of weapons created before the edict, minus all weapons created or purchased.

what do you get? Do they reproduce the weapons? Do you have to destroy your own weapons? What if you don't have any?

This is just shifting the problem forward to the endgame of a disarmed, de-billionaire'd world. In that world, currency would represent "some positive benefit to humanity," so maybe you get a portion of a park named after you, or a tree planted in your name.

I know that there are many, many problems with this idea. I am under no delusions that this would work. But the idea is interesting, and thinking about ways that we could incentivize the governments of the world to act in the interest of the world's people is probably a useful exercise.