Timing is the mechanical sequence of events between the time the trigger begins moving rearward and the hammer falls. On a double action revolver it goes like this. The trigger begins moving backwards. The cylinder stop (or latch) is withdrawn from the notch in the side of the cylinder and the hammer begins to move back. Then the hand (or pawl) begins to turn the cylinder. It is important that the cylinder stop has completely cleared the notch in the cylinder before the cylinder starts to turn. Otherwise there will be a bind in the trigger pull. As the cylinder turns, the cylinder stop is released and slides along the side of the cylinder. Depending on the make of gun, this point may vary. Then the cylinder stop snaps into the next notch in the cylinder. The trigger and hammer continue to move back slightly and then the hammer is released. This is the basic TIMING.
After the hammer falls, with the trigger still held all the way back, this point is considered "full lockup". It is at this point that the ALIGNMENT can be checked. The line up of the barrel bore to the cylinder bores has absolutely nothing to do with the timing and the two shouldn't be confused. A revolvers timing can be perfect and the alignment can be way off. And a revolver can have a severe timing problem, yet the bore line up can be perfect.
There are a couple of ways to check the alignment. With the gun in the fired position and the trigger held all the way back, shine a bright flashlight into the gap between the rear of the cylinder and the frame, aimed at the area of the firing pin. Now sight down the barrel bore and see if each bore of the cylinder is lined up with the barrel. If it's not, you will be able to see a dark crescent shape hanging into the circle of the barrel bore. The other way to check is to insert a precision ground range rod for your caliber into the barrel, the snugger the fit the better. If it smacks into the front of the cylinder when it gets to the forcing cone, you have a possible alignment problem. A very slight mismatch, if consistent on all cylinders, may not seriously affect accuracy. This is because of the miracle of the forcing cone. The taper of the forcing cone can line up bullets that leave the cylinder a little bit off. If the mismatch is worse, it can result in lead shaving and poor accuracy. There you go.
