Walk in the park.

Start by making sure that your hammer spring is on the hammer correctly, and that the two spring legs are resting on top of the trigger pin.




Next, cock the hammer down by hand until the tail of the trigger touches the top of tail trigger/tail of disco above the disco spring.
Now look at the inner side of the hammer tail, and make sure it not contacting the back round part of the disco hook, but only the spring behind it.   If the tail of the hammer is contacting the round part of the disco (not back farther and just the flat of the tail), then inner side of tail will need to be reworked, so it not slamming on the disco round part during the over cock.

Moving forward, and really over cocked the disco so it pushing the tail down and the trigger tab is touching the back of the trigger surface.
Now look at the distance from the top of the disco to bottom of the hammer.  
the gap should only be a gap of about .040" with trigger untouched ,and trigger pulled back, should be about .010" gap.
If the distance is greater, then tail of the hammer will need to be shortened.

Last one, and make sure that when you installed the disco spring in the hammer tail slot, the disco spring was installed with large coil side down into the trigger slot.  Hence should have taken a set of pliers to install the spring in the disco tail, and if it falls out when the trigger is turned upside down, then spring was not installed correctly.

So to sum it up, something may be off with the Lower receiver pin channel locations, or FCG slightly out of spec, and why the Hammer tail may need to be rework in the rig.  Also, disc spring and the tail of the disc works like a buffer on over cock, so need to make sure that the disco spring is installed correctly.  If upside down, then spring will coil bind out on the top of the trigger slot, allowing the hammer tail to really pound on the back of the trigger during hammer cycling over cock.

Another possibility is something is machined out of spec on the lower receiver, upper receiver, bolt carrier, or a combination of them.


Any chance you can post a few pics?  This may help us diagnose if it's a machine tolerance issue, or what else we may see that could be the issue or a contributing factor or factors.

1.  Specifically the bottom of the bolt carrier where the back of the firing pin area where the hammer hits it and the bolt carrier cocks the hammer as the bolt carrier travels over the top of the hammer.

2.  A top shot of the lower, looking downward on the trigger group area, specifically looking at the thickness the walls of the receiver on both sides of the trigger group area as they should be the same thickness.  If they are thick on one side, and thin on the other, then the receiver may be machined off center.

3.  A bottom shot of the upper receiver, looking upward at the side walls of the area where the bolt carrier locks into battery as it sits over the top of the trigger group.  These walls should also be the same thickness.  We see lots of off center machined upper receivers, where the bolt carrier is off center to the trigger group.



CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Maybe..

I had a friend having the same issue.

Turned out his gas port size ( carbine length gas ) was huge.  .100+ huge.

I can only imagine the cyclic speed was beating the heck out of his hammer pin.

Just a thought.

Note only major peening to the disco and trigger tail from the hammer tail, but major impact from the back of the hammer to the top of the disco hook as well.


Either Hammer/FCG is way out of spec, or lower receiver pin channels where drilled position instead.
Hence normal for the tail of the trigger to be long in some cases, but not for the back of the hammer itself to make contact to the top of disco hook.

So on that note, would send the FCG back for a refund and try another quality FCG from another vendor.  If the same problem happens with the New FCG, then problem is with the lower receiver.

Here is the lower receiver drawings if you want to start messuring to see if the problem is the lower receiver.
https://cdn.shopify.com/s/files/1/1067/2310/files/AR15_Lower_receiver_blueprint.pdf?3587052860788957110

Again, take a look at the photo I posted of his, since not only is hammer tab crashing much harder to the tail of the disco, but is also contacting the back round edge of the disco on over cock that is pushing the hammer back against the trigger pin as well.

So hammer pins is being flexed in two different directions at the same time as the hammer tail slams into the disco at  over cock (over just the tail of the disco under spring tension working as a buffer), and why the hammer pins are snapping.

If the upper was really over gassed, then it would have the bolt unlocking way too soon, extractor would be leaving the spent case pressure stuck in the chamber, and just ripping off the spent case rim on the way back.