Folks,

I need help with a King Cobra. I have a snub nose that is having a problem. When you fully load it and either try to use it in double action mode, or cock the hammer back, the cylinder does not always advance forward and jams up. If I use my finger to assist the cylinder a bit it works fine.

The strange part, to me, is that if you do it with the gun empty no problems at all - it works fine. I thought it might be the ammo so I loaded spent fired casings and no problem there either. Why only with live rounds?

What would cause this and how does one fix this?

Thanks in advance.

One possibility is grit embedded under the ejector.

It's fairly common for burned powder particles or grit to get under the ejector and get trapped between it and the rear face of the cylinder.
When the gun is fired, the grit gets embedded.
This causes the ejector to not seat fully, and this causes hard cylinder rotation.

Hold the ejector back, and give the under side of the ejector and the rear face of the cylinder a GOOD hard scrubbing with a toothbrush.
Some times the grit can be impossible to see, and very hard to get off.

Other possible causes:
Check the bushing around the firing pin hole to be sure it isn't loose, or an edge isn't sticking up and catching the rounds.  If there is a bushing problem, this MUST go back to the Colt factory.  
NO LOCAL GUNSMITHS, repairs take special tooling locals DO NOT have.

There could be an action problem, in which case, back to the factory.

Why only live rounds????
When empty there's noting there at all.
When fired, rounds are fire-formed to the chambers and are a tighter fit.
UN-fired rounds are loose and can move back easily and catch on something, OR they are just enough longer when unfired so as to cause a grit-embedded ejector to drag.

Probably cause: Dirt under the ejector.

If it's been fired much with 38 Specials, you could have a buildup in the far end of the cylinder chambers preventing 357s from fully seating.  They stick out just a little and drag on the face of the receiver.

Since it doesn't happen with the cylinder empty, it isn't dirt under the extractor (no ejector on revolvers).   This certainly can happen, but it affects the revolver whether the chambers are empty or filled.

Common things are common causes.

Are the bullets seated deep enough to avoid hitting the forceing cone ?
I am not familiar with the King Cobra, is it possible to screw the rear sight adjustment screw down far enough that the screw makes contact with the cylinder? This is quite common with S/A Rugers.

How many rounds do have through the gun? When you activate the action, are pulling slow on the double action pull? Are you cocking the hammer slow for single action? The reason for asking these questions has to do with torque. The unloaded cylinder weighs less than it does when loaded. Thus, when loaded it will require more torque to reach lockup.
Of course when the gun was new and tight it would lock up at any cylinder rotation speed. As you use a revolver the cylinder hand (pawl) will become worn. The hand (pawl) on a King Cobra is rather small for all the weight it is pushing.
Now, if the gun still does not lock up at speed, with a brisk pull of the trigger, you have a timing problem. Which in your case would still be a hand (pawl) problem. If the cylinder under rotates, the pawl is to short (worn). If it over rotates, then it would be the cylinder stop.
I had the same thing happen to me on my first King Cobra. She had about 10,000 magnum and 15,000 .38 special rounds through her. Plus, I dry fired her everyday. That's alot of cycles on the action. Most of it was done double action too, which is more stressful on the action.
I'm on my second King Cobra now and wish I had more. HTH