Is it possible to prevent a pistol from cycling by holding down the slide?

Yes, but it'll hurt like hell. My mom accidentally did that with her Remington 1100 20 Gauge once. She rested her thumb on the bolt when she fires.

But there's also the chance the slide breaks your finger first. Like my friend claims a guy once shot a Glock, and rested his thumb on the flat back of the slide. He claims it broke his thumb, and it sounds somewhat believable.

Oh another 1 often seen in movies is that a gun goes off when you drop it. (i.e: in true lies)

Are you kidding? I think the chance of a MAC-10 going off when falling down a flight of stairs is VERY likely, it is a cheap open bolt gun. I doubt it would kill all the Crimson Jihad in real life though.

It rarely happens, but guns DO go off when dropped. It's just not as common as we see in movies.

Yeah most of 'em have drop safeties nowadays, but old single actions and open bolts are just waiting to fall and pop. I've dropped my empty 1911 on my carpet before (wearing these stupid cotton gloves that slip like hell in order to avoid rust) and when I picked it up the hammer had fallen.

Luckily in a lot of cases they just fall on half-cock, but unfortunately you can't count on that.

Actually that firearm from the screencaps of CSI:NY is possible. They are sometimes called "Zip Guns" or "Gas Pipe Guns" They are not accurate, and very dangerous to mess with. Obviously the steel of a gas pipe is not the same as the steel used in a handgun, there is no rifling and using a sharp nail to set off the primer is unreliable at best.

Zip Guns were more common in the first half of the 20th century. I get the impression that they were almost a status symbol with some gangbangers. I've seen a few dating back fifty and sixty years in evidence vaults and some of them show some real workmanship. Others are pretty crude. But I would guess that a Zip Gun might help one get a better gun.

I'd love to see a myth's section :D. Just as a suggestion, tv tropes has a great collection of gun myth's/conventions; http://tvtropes.org/pmwiki/pmwiki.ph...dGunplayTropes. I'm not sure how accurate they all are, but they the few i've read seem alright to me.