How to destroy a gun...

Bring it to your local PD or FD, call first. Alternatively, wait for a "Gun BuyBack".

torch.

Wait for a gun "buyback" and at least trade it for something useful. Maybe even some $$ to buy rounds or towards a new gun.

Remove the firing pin, chuck it out and then stick the pistol in a drawer until the next buy-back.

hack or band saw + lake.

Or..... you can shoot it with a rifle

Is there a "complication" if you wait for a "BuyBack"?

Torch it. I would not support any gun "buy-back" program by selling it to them(they should all wither and die from attrition).

I have to ask why would it need to be destroyed? even saturday night specials make great paper weights without the firing pin

Mix up cement in a bucket and turn the offending weapon into a rock

As said a "buy back" Now to disable it you can remove the bolt/firing pin. You could put some JB Weld in chamber (so no bullet could be chambered)
Most guns you could disable without it "looking" unuseable without working action. (I.E. wall hanger)
If you weld you could run a bead in chamber edge (so action could be worked but impossible to chamber rd) Many options.

The way I have seen them destroyed is to cut the actual gun part (receiver or whatever part of the particular firearm that is considered the "gun" ) in half . For example AR 15 the lower receiver is the actual "gun" .

Torch

Take out the firing pin and make a paper weight.

Chopsaw, bandsaw, torch, welder (weld the bolt to the barrel and/or fill and plug chamber with weld).

Then use the cement to turn it into a rock (I like that one too).

Fill it with JB Weld. and hang it up

another gun... gives a new definition to shooting a gun.

I second Evnldr's idea to fill it with JB weld.

Take out the firing pin and hang it on the wall (as already mentioned).

Take a bunch of Unique powder, and grind it down with a mortar and pestle.

Place a magnum primer on a spent cartridge. Fill the cartridge to the rim with this powder, then smash it down, and repeat. Seat the heaviest bullet you can possibly find.

Stick this cartridge in the gun, and stick the gun in a vice. Hide behind something heavy and rated to protect against metal fragments traveling at high velocities (shrapnel) and pull the trigger with a lanyard.

*Note: This is probably an extremely bad idea, and I am not to be held liable for any loss of life, limb, or property should anyone actually do this*

Cut it in half, torch, hacksaw, sawzall, whatever. Or, beat it with a sledgehammer.

Done buybacks. Also have shattered an alloy framed .22lr rifle and a polymer pistol frame with a sledgehammer.

I think I see the need for a new "Cash for Clunkers" program!!!

Just bring in your old high mileage gun, and let the government give you a few thousand dollars towards the purchase of a newer, higher efficiency firearm

In all seriousness, I think the answer to this question partially depends on what you mean by "destorying" a gun... You might destroy the gun so that it is no longer functional, but still looks like a gun (wall hanger). In this case you'd probably want to look into welding type options, or just removing the firing pin for a more reversible fix. On the other hand, if you are trying to simply destroy the gun, you might go as far as to cut the receiver in half. Finally, if you are trying to destroy it to meet the requirements of some obscure statute regarding having a gun destroyed, then your method will depend on the verbage of such an obscure statute!

As silly as gun buy backs are you can make money on them if you have enough SNSs that you got for cheap.

8 pound sledge?