Verify that both receivers are .154 size, and not one a .154, and your Colt a .171 channel receiver instead (read so you don't have .171 FCG parts on .154 pins).

As for the check, with the trigger pulled all the way back, hammer should cock all the way back/down, and be cocked to the disco.  Releasing the trigger should allow the hammer to release from the disco, and have the trigger front primary sear catch the hammer via the hammer back cam sear.   Another trigger pulls should release the hammer from the front sear.

If the FCG pin sizes are correct for the parts/receiver, then double check the disco spring to make sure that you have installed it large coil side down into the back trigger slot.

The rest of the springs installed,

Give me a closer photo of the FCG so I can check the spring installs.

Give me a close up of the side of the receiver so I can see the pins in the receiver channel. Around 2008 is when colt stated to change over to the .154 parts, instead of the .174., and I just trying to remember when Colt switched over to the integral sear block to determine if the receiver should have .154 channels instead of .174.   The reason behind this is if you have a .154fcg in a .174 channel receiver, the slop is going to cause the problem every time.

As for the half cock and the hammer catching the bottom of the carrier on the way forward, it normal.  The colt semi system has an anti follow through in place,and unless the hammer is cocked back to either the primary or disco sear, the L cut above the hammer firing pin contact pad is going to catch the FP collar to prevent the bolt from locking home.

Lastly, give me a photo of you hand cocking the hammer back so the back sear of the hammer, is as close as you can get to the disco sear with the trigger untouched. I'm looking for the free gap between the two.

Side/down'ish view of the hammer being cocked back to get rear sear as close to the disco with the trigger untouched please.

Also, double check the channels in the lower receiver (photo please of the side of the receiver so I can see the pins against the sidewall of the receiver.  Granted that you know that the FCG is .154, we just need to confirm that the receiver channel holes are .154, and not .174 instead (causing the slop).


Also, with the straight down view, it looks like the front/bottom of the disco is not setting flush against the front/top of the trigger. Are you sure that you have installed the disco spring large coil side down into the trigger slot under the tail of the disco?

I say this since the disco seating correctly against the front/top of the trigger is what dictates the free gap between the disco and hammer rear sear, hence sets the hammer release of the hammer from the disco at trigger release to get the trigger front sear in the correct position to catch the hammer via is primary sear on the bottom back lob at reset.

It could be an illusion, but double check this to confirm that the front of the disc is making contact with the top of the trigger, and not the disco maxing out against the top of the trigger at the back corner edge instead (cam'g open the disco, and causing the advance release of the hammer on trigger release reset).

No worries,

As I stated, along the way when Colt switch to the small pin FCG, they switch from the protruding sear block, to just not fully machining out the back shelf to prevent an auto sear from dropping in that way instead.  The fact that you had the protruding block, is what kept sending up red flags when you where stating that the FCG for the rifle was .154.
Note, there may in fact be some .154 colt receivers with the old style block, just I have not run across one yet.

Both ways keeps the receiver from becoming a MG host, but at least the current way allows for after market NM to be installed, since with your old style block, you have to do a bit of milling to get part of it out of the way.