Guys,

I have a lower that I milled identically along with two others using 5D router jig.  One lower will not allow the trigger to function properly. At first I tried drop in trigger and it would not function. I then installed a known working mil-spec trigger from another lower and it seems like the hammer can be set at "half cock" and will work pulling the trigger but when the bolt cycles the hammer is pushed further down and is fully cocked but will not fire by pulling the trigger.

Before I start filing, stoning or removing more metal can anyone provide any insight into this?  I am assuming this is a common problem and knowing what to address initially may help me save some time.  

Thanks.

It sounds like the trigger pocket has been machined too shallow.  The bottom shelf may not be allowing enough clearance for the trigger to pivot fully and release the hammer.  It's easy to miss... Not enough downward pressure during the final machining pass... tolerances in the router...  a loose mount...  even when we think everything is identical, a variable can pop up.

Thanks for the info. I read a bit on the forum and saw that noted but wasn't exactly like my scenario. I'll take your advice and start there.  

Pocket could be a little shallow or the front wall might be too thick or radiused too wide and binding at the edges.  Alternately your fcg pin locations could be off.  One time i had a small shelf at the bottom of the lower where i had milled the final dimensions with the bit slightly off the floor - thus leaving material at the front floor which interfered with the fcg.  

Also make sure the trigger bar isnt caught side to side or front to back in the trigger hole.  

Finally make sure the pins are seated correctly and the fcg moves freely (not binding against the walls) - little too tight and they can get stuck and not move.

Dont adjust the fcg - fix the issue on the lower.

UPDATE:

So I purchased a LUTH-AR standard 2 stage trigger and an ambi-selector switch. I decided to fit the selector and had to hone some of the hole to get it to fit right. I then decided to just try the new trigger to see if I could replicate the problem I had with the other 2 triggers. Amazingly the trigger passed the function check. I am impressed with this trigger for only being $40. Good 1st stage takeup and crisp break.

lesson learned...handfitting and swapping components works when machining tolerances are lacking.