I'm going to say that the gun has been dry fired extensively and there are some parts missing from the firing pin block. These small parts are required to depress the plunger on the bottom of the slide and allow the firing pin to move forward in the firing pin channel.
When the plunger isn't depressed the firing pin is captured and can't move forward. When you dry fire it in this condition it like hitting a piece of steel between a hammer and anvil.
Your gun should have two small parts in the frame that rotate on the hammer pin and sear pin. They work off the trigger shoe presssure to depress the plunger (firing pin block) on the bottom of the slide.
Field strip the gun and while holding the frame pull the trigger (ease the hammer down). With the trigger depressed you should see a small lever lift up and to the right of the disconnector on the rear of the frame. If no little lever pops up that's your problem.
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