Policy still matters even if guns exist. After all, murder is still illegal even though murderers exist. Building a bomb is still illegal even though bombs exist.

The tricky part with the US is the already vast supply of firearms circulating. Can't do much about that.

But, I would think, stopping or reducing the sale of guns right now would still have an effect. We already somewhat regularly try to reduce the sale of guns via policy, mostly to people we think are potentially dangerous.

But, I don't know exactly how much that has helped, or will help. What I do know is there is definitely variance in gun violence. Both across nations, but also across states in the US. So, something is behind it.