This is an image of an original SAA hammer that has been reworked to have new sear surfaces. The geometry is exactly the same as on the original Colt SAA. This is the classic fixed firing pin. It is solid and does not move.
There are three notches that the trigger sear can rest in. The first is sometimes called “rebound”. It is the deepest of the three. You should not be able to pull the trigger in this position. When in this position, the firing pin is away from the primer. It can not go bang from a trigger pull nor from striking the back of the hammer. To cause the firearm to go bang you have to strike the hammer hard enough that it breaks parts of the trigger or sear surfaces.
This is the recommended hammer position if carrying with a round under the hammer.
If you pull the trigger and the hammer at the same time such as the hammer rotates back enough to clear the first notch but not enough to get to the second notch and let go of the hammer there should not be enough force to fire the gun. No bang.
The next notch is the half-cock notch. Again the trigger is locked in place. You can not pull the trigger without breaking something. Striking the hammer does not cause the gun to fire. Hitting the gun doesn’t cause the gun to fire as the first notch will catch the hammer before it strikes the primer.
The hammer in half-cock can not be made to fire the gun without breaking things. If the hammer is pulled back and the trigger is pulled and the hammer is released before it gets almost to the final sear the gun does not go bang. Again, you have to pull the trigger to allow the hammer to fall all the way and with enough force to ignite the primer.
The final notch is the actual sear. At this point the gun is fully cocked. Hitting the hammer or gun will not cause the gun to fire. The trigger spring (not shown) will push the trigger sear back against the hammer and the hammer will catch on the half-cock or rebound notches.
In order for the gun to fire you must pull the trigger.
The final possible position is with the hammer fully forward. In this situation striking the back of the hammer firmly enough will cause the gun to fire.
This is the type of hammer the gun Alec Baldwin used to kill the camera woman.
Here is a transfer bar system. This revolve has at least a full and half-cock notch. The big difference is that the front of the hammer is smooth. NO firing pin.
This is how many modern hammer fired firearms work. When the trigger is pulled a bar extends up to cover the back of the firing pin. The hammer is released as the sear is disengaged. The hammer falls and hits the transfer bar. The transfer bar strikes the back of the firing pin. The firing pin then impacts the primer igniting it.
When the trigger is released the transfer bar drops down. With the transfer bar down the hammer doesn’t touch the back of the firing pin. Hitting the back of the hammer just drives it into the frame. No bang.
This is a very safe arrangement. Even if the hammer was at full cock and the gun is dropped it will not fire as the transfer bar will be down.
It is “safe” to carry with a round under the hammer.
The issue with this style of SAA is it doesn’t look right to people in the know. That firing pin spur is very distinctive.
So which of these did Alec use? Neither.
He used a hybrid system. Instead of a fixed firing pin as shown in the top the firing pin is actually attached to a small spring and floats within the hammer.
When the hammer strikes with enough force the inertia against the firing pin causes it to continue forward to strike the primer hard enough to ignite.
This system has all the same safety features of the original SAA but with the added benefit that the firing pin does not touch the primer when the hammer is full down.
In addition this style requires the system to be more finely tuned. If the hammer spring is to weak compared to the spring holding the firing pin in place the gun doesn’t go bang. If the firing pin spring is to weak it is possible for the firing pin to touch a primer without the hammer falling from a full cock position.
It is also possible to fire this style with a strong enough strike to the back of the hammer in just the right angle.
What the FBI report said is that the gun was tested and the following was found.
The gun would not fire if the hammer was not all the way back.
The hammer would not drop unless the trigger was pulled.
The gun would not fire during normal drop testing. I.e. If the hammer was fully down, at half-cock or in the rebound notch it would not fire.
In order for this particular gun to go bang it would require that the hammer be fully cocked and the trigger pulled.
I have personally laid hands on all three of these types of SAA revolvers. With the revolvers I own I tested with a case with primer only in the chamber.
I believe that Alec killed that woman by pointing a loaded firearm at her, cocking the hammer, and pulling the trigger.