I saw something in a documentary about mistakes in forensic science that related to this… (VERY SQUEAMISH READERS STOP HERE…)
They described this one case where a woman committed suicide, but the coroner had originally thought she was shot by her husband because it seemed the smaller entrance wound was in her back. It turns out the larger wound in her stomach was actually the entrance wound. Apparently, if the gun is held tightly enough against the skin, the expanding gases from the barrel follow the bullet into the newly created hole, ripping the wound open. The gun itself did not appear to be damaged.
If memory serves, the gas pressure inside the barrel upon firing is somewhere in the neighborhood of 20,000 PSI. Sticking your finger all the way into a rifle or pistol barrel would likely explode the gun and your hand. Anyone who’s seen a fired bullet knows how deformed they get when they hit anything, and if it gets stuck in the barrel, the 20,000 PSI pushes out elsewhere, namely the weakest parts of the enclosed space. However, your finger would have to be small enough to fit in the barrel. My little finger is more than half an inch wide and could only fit the tip in a .44; I doubt it would work if I were insane and tried it.
I can only guess what pellets from a shotgun would do if plugged by a finger, but I suppose with the wadding, it would generally tend to explode the gun in the same fashion as a rifle. I would expect that it might take less backforce with a shotgun, as the barrel typically has a much thinner wall than a rifle. And since shotguns are more powerful as point blank range, there’s a good chance it would still kill you anyway.
But, based on the suicide above, I don’t think holding your finger tightly over the end of the barrel would have any effect (on the gun that is). If you are indeed close enough to stick your finger in the gun, you’d be better off pushing the point of the gun away from you. Or give whoever it is what they want if you care to live.