It would have to be structured as a voluntary turn-in with a benefit that people would be crazy not to take advantage of (e.g. some gigantic tax relief).
Yeah, don't try or even think about it, not worth the risk...
But serious, why not start a voluntary buy-back program and see if gun owners respond to that. Or how about a law that makes it mandatory to offer the gun/ammo to the government when a private owner wants to sell? Private sales are a huge problem: no background checks. Stopping them while still giving owners a chance to sell could do some good.