Just make them illegal. Confiscate/destroy as they surface. Introduce buyback programs. Those who want to can hide them.
If you think about it for a minute.
If you can't just go buy one at Walmart.
Imagine what happens if the only guns you can get are what you have, and what's on the black market.
Imagine if it's HARD to get a gun.
They want that to happen. They dream of it.
Which is why they're hoarding guns.
Let their guns rot.
Don't shoot the beast. Starve it.