Ykhjuy

Suppose that someone is pointing a gun right at you.

He's not really convinced about shooting, so you talk and manage to get very close.

You keep talking and you can actually put a hand on the gun, but he's not letting it go.

Now:

• If the gun is a revolver type:
  
  
  
  
  • If the hammer is already all the way back, would it be possible to prevent the gun from firing by putting your thumb between the hammer and the body of the gun?
  
  
  • If the hammer is not already set, the cylinder has to turn to be able to shoot. Would you be able to prevent firing by having your hand firmly on the cylinder?
  
  
  
  


• If the gun is a "slider" type (not sure about the actual type name), would you be able to prevent it from firing by having a firm grip on the slider?

For a revolver type, blocking the hammer or the cylinder can prevent firing. I think these are two of the reasons why a revolver can get jammed and fail to fire.

For an automatic or semi automatic gun, the slider moves after the shot has been fired to allow the expulsion of the muzzle. So, even if you were able to hold it in place, the bullet would already be travelling to its target.

In both cases, considering that the gun won't be calmly in place but its holder would probably attempt to shake it free from your grip, I think it's, to put it mildly, a rather daring attempt.

I agree with most L.Dutch says, including the point that only skilled and heroic characters should attempt this. One little issue regarding the semi-automatic pistol:

Racking the slide back and holding it open should prevent fire. Pulling it back ejects the unfired round in the chamber, holding it prevents the chambering of a new round. It might be possible (but painful) to jam a finger in somewhere to prevent the grip from slipping.

(Disclaimer: I don't know that from personal experience. Sounds like a way to the Darwin Awards. But the mechanics seem straightforward.)

A somewhat silly solution:

A good bunch of handguns have a considerable amount of empty space behind the trigger. If you can slide your finger there, you'll prevent the trigger from being pulled at all, so no firing can take place.

TL;DR : Most on the time, yes.

It is actually very possible for both type of handguns.

A semi automatic handgun (the one you call slider) can, on its own, fail to go into battery. I'll let you look for pictures but the point is that the slide did not complete its travel to "close", even by as few as 1 or 2 millimeters. The striker then can't hit the primer on the bullet therefore gun not firing.
Applying pressure on a loaded gun with your hand to get slide slightly back would effectively partially disarm the firearm.

Blocking the hammer on a classic revolver pistol will have the desired effect. Why do I precise classic ? Because some revolvers, mostly subcompacts only have internal hammers which can't be grabbed.

Another solution could be engaging the safety or blocking the trigger but those are even more complicated and risky.