When the gun fires and the slide moves rearward the disconnect arm rides across that little hump on the slide and causes the sear to move forward so it is ready to catch the striker lug when the slide locks back up. If you take the slide off and pull the trigger, push down on the disconnect arm and you will see the sear jump forward. As you release the trigger you will see the small ramped part of the trigger bar move forward and resetting on the sear. That is the click that you hear.

Bill

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Remember the Alamo!