Take the gun. Damage the gun, take out the firing pin. Take out the spring.
Damage the hell out of the gun, make it misfire or better yet blow up
Yes, that was the question.
Do you have an answer?
Pour something highly corrosive into the barrel or trigger assembly (forgive my ignorance - wherever you’ll find the most moving parts)?
About the only thing that would be universally effective would be to squirt a tube of superglue into the trigger assembly. No one would be the wiser until they tried to pull the trigger.
Yeah, messing with the mechanism sounds like a better idea than messing with the explosive part. Especially if you end up with the gun pointed at you.
Correction: The barrel may explode if the gun is shot with a plugged barrel. It’s only in the movies that this happens every time. If the shooter is lucky, the bullets will still exit the muzzle without the gun blowing up in his/her face. Example. Some service pistols, like the H&K P30L, have been designed to withstand a shot with a bullet stuck in the barrel without blowing up. However, the barrel will be ruined and must be replaced.
I’d go for what the others who seem to have some knowledge about guns recommend: Remove the firing pin. It’s unobtrusive and the gun is just as useful as another piece of steel, wood and plastic.
It is faster to just take the whole revolver and throw it in the bin.
I think I’ve seen this on a documentary…just put your finger in the business end of the barrel and when fired the gun will splinter like a reed, possibly backfiring.
The name of the documentary, IIRC, was “Loony Toons” 
You’d really need knowledge of disassembly for that particular firearm to take parts out quickly and easily. Otherwise you’d need extra equipment like superglue or spray foam insulation. Revolvers or bolt-action rifles might be an exception.
Overall I think it’s easier just to put the gun somewhere inaccessible.
I agree removing the firing pin would guarantee the gun would not fire, however, it would take most people a LOT!!! longer than “a few minutes” to make the gun inoperable by taking out or damaging the firing pin, which is what the OP is looking for. Something fast. In fact, I really do not think most people could even do it (assuming they even know what a firing pin is).
Removing a bullet and jamming it into the barrel is the quickest and easiest thing to do and able to do it without needing special tools.
If you are soooooooooooooooo worried about the gun not blowing up with a bullet lodged in the barrel, then jam 2 bullets, jam 3 bullets into the barrel.
Then just remove the slide stop pin. Not immediately noticeable, can be done in less than 3 seconds and renders the gun completely useless. Jamming bullets down the barrel requires tools, unless you are a lot stronger than I am! Prying a bullet loose from its brass takes a good pair of pliers, minimum.
I still don’t understand why you don’t throw the gun away, or keep it and shoot the guy. It’s kind of like the bad guy and James Bond. Don’t show him where the self destruct button is and then have an incompetent stooge lead him away, just shoot him.
Alistair MacLean was notoriously misinformed about some things, but his protagonist in When Eight Bells Toll filed down the firing pin with a three cornered file.
An episode of Burn Notice had an example of this. The bad guy wanted to disable the good guy’s weapon but not alert the good guy it was busted, because the bad guy was under cover as the good guy’s client, but was really an assassin, using the good guy to hunt down the target. So the trick was to steal the trigger spring while the gun was in the glove box and the good guy went into a gas station/store. Worked like a charm.
The good guy, Michael Weston, was able to jury rig a replacement using a bobby pin, but it only gave him one shot.
The scenario is that you need something from the bad guy so you need to pretend he has the drop on you/think his gun works so he thinks he has the advantage.
Another example was from Die Hard when Bruce Willis handed Alan Rickman the unloaded gun, and Alan didn’t check himself.
Ever tried to jam a bullet down a barrel? No? Thought so. You see, the rifling cuts into the bullet, and the friction makes that a not particularly easy endeavor. Then try doing that with two or three bullets. Good luck, it’s gonna take you some time and cost you some sweat. Unless, of course, you’re dealing with a flintlock musket, and even then you’re gonna need a ramrod.
On afterthought, I’d go for removing the entire gun rather than the firing pin, as others have suggested. Fastest and easiest solution.
Originally Posted by Susanann
jam 2 bullets, jam 3 bullets into the barrel.
Naw, I have a hammer. Hammers are pretty easy to come by, and I can easily pound a bullet into a barrel with a hammer in about 15 seconds. If you cant find a hammer, then use a rock and a nail. Nothing to it!
As far as 2 or 3 bullets, that was only for those who think a barrel completely jammed with 1 bullet is not enough. I personally would only jam 1 bullet into the barrel, 1 bullet completely and tightly jamming the barrel is usually enough to prevent the gun from shooting properly.
Which wasn’t the question.
Why do some Dopers feel the need to answer questions that very specifically were not asked?
The Bad Guy may well have an arsenal of, say, 50 weapons. That isn’t all that unusual. IIRC several Dopers have collections of that size. A single person on foot is not going to carry 50 handguns, much less 50 longarms.
Even if there is only one longarm, where are you going to remove it to?
I can think of a million places to hide a cylinder of slide in a normal house where nobody would find it for months. A firing pin or spring could be flushed down the toilet. Hiding a longarm is going to take longer than disabling it.
Removing weapons is not always easy.
It all depends on what you mean by “shooting properly”. Yes, precision will go south, the barrel will be ruined and should be replaced and recoil will be excessive, but there’s no guarantee that the bullets won’t exit the muzzle still able to maim and/or kill. At a range typical for handgun combat, the bullet(s) may just as well hit you as not even if the precision of the gun is busted. Yes, jamming the barrel is a Very Bad Idea for the gunowner, because the result is a ruined gun and a much-higher-than-acceptable risk of the gun blowing up and maiming the shooter. The problem for the target is that it’s just as likely that the gun will still fire somehow. For modern guns, it’s probably more likely that the gun will not blow up.
In short: If you’re the shooter, don’t jam the barrel. That can be nasty. If you’re bing shot at, don’t bet your life on standing in front of a firearm with a jammed barrel. That gun is still pretty damned dangerous at the business end. In fact, guns are generally pretty damned dangerous.
ETA: Some guns, like the Mauser M98 or John Browning’s Colt 1911 were made to work under pretty bad conditions, like trench warfare. They were made to not blow up from small problems. Pretty damned good engineering, those things.
Why do some Dopers not read the OP? I doubt “removing the cylinder and throwing it in the bin” is going to go unnoticed on a revolver for very long.
I often wonder why some of us answer, “What is your favorite single [insert subject here]?” with a list.