For those wondering if their guns lock has a "fly" in it so a set trigger will work, here's my method.
Bring the lock to the full cock position.
Put your thumb on the hammer spur and pull the trigger, letting the hammer slowly fall just far enough so it won't go back into a locked full cock condition. Then,
release the trigger.
Lower the hammer slowly. About the time it gets to the half cock position your lock will do one of two things. Either the sear will engage the half cock notch and totally stop falling or, hopefully, it will seem to start to stop and then it will continue to fall down to the fired position. At this point, it might actually quit falling but just a slight push may start it falling again.
If the hammer ends up falling all the way to the fired position, the lock has a fly in it.
If it goes into the half cock notch and won't fall further, even with a little push, the lock does not have a fly in it.
Of course, a person can take the lock out of the gun and look to see if there is a fly in the tumbler. It will be very small and it will be located at the mouth of the half cock notch in the tumbler.
Because most locks have an internal bridle that covers much of the tumbler, it can be hard to see if a fly exists or not. The best way is to bring the lock to a full cock position and turn it upside down. You should see the end of the "sear" (the bar with the leg sticking out of it for the trigger to push against) in the full cock notch. Looking forward on the underside of the tumbler you should see the half cock notch. Look for a very small little steel thing that seems to be blocking off the half cock notch. Using a sharp pointed thing, if you push on it and it moves, you have found the fly.
This fly is there to block off the entry to the half cock notch so that as the tumbler is rotating when the hammer falls, the nose of the sear cannot enter the half cock notch. Without a fly in the lock, most set triggers won't work.
Have fun.
