Balzac72,
Before he takes the rifle out again, you need to perform a function test on the rifle.
Chances are, the rifle is going to fail the tests. Since you stated nothing about tuning the disconnector, the rifle will still have a problem.
HOW TO TEST.
Swing open the lower, and hold the trigger back.
Then cock the hammer to be retained by the disconnector.
Push the hammer all the way down, then let your thumb slide away(to the side) and see it the hammer is retained by the disconnector.
The hammer needs to hit the disconnector full force to check the engagement of the two.
The ever so slowly, release the trigger until the hammer is released from the disconnector, and retained by the trigger sear.
Here is where you check the real timing of the disconnector. If the disconnector releases too soon in the return stroke, the hammer will be released, and miss the trigger sear. Hence releasing the hammer to fire another round on the return stroke.
Since the normal reaction when firing a rifle is to squeeze the trigger. Then during the recoil to allow the your trigger finger to slightly dance on the trigger, with a badly timed disconnector, this will cause the auto firing.
Since you stated that the hammer never bound up, then either the hammer was bounced off the disconnector at carrier impact with the barrel/receiver, or the hammer was released on the forward stroke of the trigger and missed the sear.
Also, since I see that you wish to tune the rifle, instead of just changing out the FCG, here are the instruction.
The function of the disconnector is to retain the hammer until trigger resets. By retaining the hammer, you want the release to be just short of trigger reset(trigger coming to rest al the way forward).
The disconnector can be tuned by removing metal from it where it contacts the trigger. By removing metal from the front foot where it contacts the front of the trigger, you are caming the disconnector forward, which in turn, makes the disconnector rotate farther forward. Hence, this requires the disconnector/trigger to be father forward before the hammer is released the disconnector. The correct timing will allow the trigger sear to cam farther forward, and be in the correct location to to retain the hammer sear at release from the disconnector, instead of missing/deflecting off the hammer sear.
When removing metal from the disconnector foot, remember that the disconnector is attached to the hammer at a pivoting point. Which means that when you remove metal from the front foot, you will need to remove it in relationship to this the pivoting point. If you just remove metal at the same angle of the current foot, them what will happen is that the new contact angle will be wrong, and only the tip of the foot will rest on the trigger's top edge.
This leads to the new contact edge wearing down, and due to the added caming of the disconnector forward, the disconnector does not release the hammer, even when the trigger is at full rest.
Also, if the hammer sear is worn/tweaked, then this will also have to be considered in to the adjustment. If it is too worn/tweaked, then all the adjustment of the disconnector is not going to fix the FCG, due to the sear not having enough of a edge to allow the trigger sear to retain the hammer.
Or,
as Royce would say, just replace the FCG. For the $49.95 that it will cost you, it's a lot cheaper that having to deal with the BATF or replacing the receiver if you have a Kaboom.
Dano