The bottom of the hammer is cam'd oblong,

With the hammer unlocked, it cams the front of the trigger down and will not allow the selector to be set to fire.

when the hammer is back, it allows the back of the trigger cam down, and allows the selector to be set to safe.


The glitch you are having, is the area just above the sear on the hammer, and it too proud isntead.  Hence with the upper open, as you cock hammer just enough for the trigger to catch the sear, the back of the trigger should drop down.  As you over cock the hammer to make the tail of the hammer touch the the tail of the trigger (forget the trigger bump of the disco locking on to the back sear of the trigger, the front top of the trigger should not be contacting the hammer above the sear isntead.

So, lock the hammer back on the trigger, put the selector safe, then cock the hammer all the way until it tail touches the top of the trigger, and make sure that the front top of the trigger is not binding up on the areas above the hammer sears.



Note the releif gap of the area above the trigger sear, to the front of the hammer.  This allows the hammer to over cock, without it causing the back of the trigger to be cam'd upwards.


As for solving the problem, the easy way is just a replacement hammer or FCG, but if push comes to shove, you will see the area well above the sear where the front of the trigger is binding up since the upper relief was not ground high enough by the hammer cocked to trigger, selector set to safe, and cam the hammer all they back so the front of the trigger marks the areas on the hammer via binding, and you can remove some metal on the hammer where its binding (stay well away from the top of hammer sear shelf, since the metal that needs to be removed on the hammer is way up above it instead).