It depends on what size gun you want to use and what style, plus I'm not sure what sound you're looking for.

There's the 9mm smith and wesson
colt 45
beretta 92
Ruger

there's double action and single action revolvers
semi automatic pistols
break action pistols
bolt action pistols
there's a nice range of clabbers as well

there's a 357.
a 44.
a45.
a 480.
a 454.
and 500.

there's also ammunition types
Jacketed hollow point: for self-defense
Jacketed round nose: for defense practice
Jacketed soft point: for hunting small game
Unjacketed wadcutter: for target shooting
Jacketed flat point: for practice, target shooting
Unjacketed round nose: for inexpensive practice

and that's only a little about handguns.
I'm not sure if you wanted to use a rifle for this scene

Do you know anything about guns? Fill in the blank if you do.

The story begins...

Chapter title:
Life is a gamble.

Every morning Jack would load one bullet into his 45 and spin the chamber. Then he would place the gun to his forehead and close his eyes. He never pulled the trigger. He wasn't quite ready to die just yet. Then he heard someone behind him cok the chamber of a _____.
(fill in blank with the name of a gun)

"Go ahead and shoot." he said, but then he adjusted his aim and fired. The intruder fell to the floor, hit strait between the eyes.

Nope, he wasn't quite ready to die just yet.

Wow I would just delete my reply......but I won't....only because when I do something stupid I own up to it. You obviously know that. And what you were asking for was nothing at all what you recieved and all I did was kind of reask your question. What you need in place of my answer, the "45" or "pistol" which both of those two things are already in the "paragraph" is a "name of a gun" and the names which are mentioned in this "bulletin". Wow....sorry I'm a dimwit who doesnt read everything before he replie sometimes. From one person to another, I apologize. And I know nothing about guns, and I see why you asked someone who did to respond....probably making it so you didnt get a hundred replies like mine. You were smart, I was not.

Thanks for your suggestion. It was not dumb. I considered it, but I want to name the gun because the guy in question is an expert in guns and he is familiar with the sound of that particular gun. Perhaps he even knows who the guy behind him is and what kind of gun he uses and that's why he does not hesitate to re-aim his 45 and shoot.

Here is an interesting story: I was in a truck with my brother and his two kids who he has well trained in how to load and shoot guns. They had an automatic under the back seat. Then a song came on the stereo that was a rap song. In the background of the rap song there was the sound of someone co*cking an automatic pistol. It sounded just like the kids in the back seat were co*cking the pistol.

My brother turned around to yell at them and ask them what the hell they were doing with that gun.

The gun being co*cked behind you is an unforgettable sound.

To be honest...I don't really think the chamber ever c**ks. It's the hammer, even on semi autos that c**ks. Aside from that, a Colt always gets peoples attention. But Taurus makes some fine smaller framed shortrange pistols, perhaps a .22 mag with hollowpoints filled with mercury, sealed with wax? Or some other poison in a hollowpoint...I just suggested a .22 due to the concealability, accutacy at close range, and the spare ammo would not be too bulky if the assassin needed more. Good luck with the screenplay. Any more questions about guns and the like I'd be happy to give a shooter's answer.

See that's how much I know about guns. I don't know what part co*cks. LOL

The .22 mag sounds like a good possibility. So should it read:

"Then he heard someone behind him co*ck the hammer of a 22. mag?

Is a .22 mag an automatic? I thought it was a chamber in an automatic and a hammer in a 45. But I know nothing about guns.

Well I think you're aiming towards a revolver type pistol where the (hammer) is being pulled back.
Something you see a lot of in old westerns and some mafia style movies.
What that does is to make the firing rate a tad bit faster.
So instead of you having to pull the trigger and the (hammer) pulling back then releasing to fire the bullet, the (hammer) is already in the ready fire position.

For a more modern style pistol which could be used as an assassination
I would recommend the Springfield XD
I've personally shot one of these as my step father owns two.

it's a very universal handgun which police and military use.
it has the option to change barrels for higher and lower caliber.
it has attachments for laser and flashlight that sit on the front underside of this pistol for tactical shooting, and like most pistols also has the option for a silencer piece which an assassin would use for a stealthy kill.

If I knew what the mentality or style of the assassin in your screen play, I could probably narrow it down to a few types of handguns or even rifles.
If the assassin has military background, you might even consider a blade oppose to the gun.

I hope this helps a little