Once you get the gun away, what do you do with it?

You can bring them to the local police. That’s legal and permanent, how safe it is to put more guns in the hands of the police might be a question.

You can turn them into the police. When I go out hunting I drop off any leftover ammo at the police station since I am not comfortable with it in the house.

You can sell or give them to someone who wants them. You could also let the person keep them but render them inoperable by removing the bolts, firing pins etc…

Turn them into the police.

And what if you’re worried about the gun for some other reason, such as that it might not have been legally owned in the first place?

Then toss it in a lake after wiping your prints.

I’ve always thought the wooded median between the Northbound and Southbound lanes on the interstate would be a good place to hide something you didn’t want found. No one ever goes in there.

You could say you found it in the room of a former housemate while you were cleaning out the house. I think if you are handing it in, they won’t question you much as to provenance.

@Jeruba You can still just take them to the police dept. Please do not toss them in the lake or in the median of the highway.

I live near the Mississippi, so, no problem with disposal.

I also know trustworthy people that would pay me cash for a gun, untransacted.

I would seriously consider @ARE_you_kidding_me‘s suggestion to permanently disable the gun and allow the owner to keep it. At least that would keep them from seeking a replacement, and be a clue as to their mental state.

Safely?
Unload it. Then let the kids play with it. Nothing much safer than an unloaded gun.

Legally?
Give it to the police so they can sell it to someone else at auction.

Permanently?
Guns sink. So @ibstubro‘s river is as good as any permanent solution. Unless you’ve recently spied Jacques Cousteau slummin’ the local catfish barge.

Unload it.