I just built a new 300 blackout with aluminum 80% lower which I drilled and milled to blue-print specs.  Purchased 16 inch upper from hardened arms with 16" barrel. The upper came with carbine length gas tube. I used hand loaded Hornady 125 grain sst with 17.5 grains of H110 for my test firing.
My problem is when I shoot rifle the first time, the bullet chambers and fires, ejects and loads in a new round fine but the trigger will not "pull" for the next round.  I swapped the upper over to a new frontier 223/556 polymer lower and the gun works fine!  I took the trigger assy apart and checked disconnector spring and the big side of the spring is down.  The trigger assy (plastic/abs) is from an old polymer lower. I tried the function checklist and the disconnector seems to work according to spec.  I have read the entire troubleshooting thread and am sorry if I missed something!  Any suggestions?

UPDATE: I broke down and brought my rifle over to New Frontier Armory and had their gunsmith check it out....this is what he found.

1.  I was using a metal selector switch with polymer FCG.
2.  trigger mounting pins were the wrong size (they were for metal FCG)
3.  Polymer hammer was damaged/misaligned.

all bad/wrong parts replaced and gunsmith deemed everything serviceable, they only charged $35.00.  Gunsmith recommended replacing FCG with metal one as soon as possible (just like you guys said!)

Sounds like a disconnector issue.   Try a non-polymer fire control group.

http://palmettostatearmory.com/index.php/ar-15-05/lower-parts/parts-kits/palmetto-state-armory-fire-control-group.html

I swapped the 300 blk. upper to a polymer new frontier lower and it works fine.  Why would the new aluminum lower keep the bolt from engaging?

I would duble check the trigger/hammer pin holes. You may be off just a hair.

If you take the lower off the upper, hold the hammer back with your hand, put the lower on fire, pull the trigger and hold it back, while slowly leting the hammer go forward. recock the hammer, now release trigger. rinse and repeat.

if the hammer was released the 2nd time, more than likely somthing is a miss with your conpletion with your 80% mill out, and I would start by checking the hoels that hold you pins for both trigger and hammer, (both sides of the lower).

tried what you suggested and the hammer stayed engaged both times.  When I release the trigger slightly you hear a small click and the hammer moves forward just a bit but does not slam forward.  From what I read this is supposed to happen.

yes, that small click and hammer moving fwd just a hair is the hammer coming off the disconector and catching on the trigger sear.

If it functions correctly by hand, try puting your rifle together, and try dry fireing it. do not release trigger after you dry fire it. pull the charging handle back and then allow it to go back into battery while still holding trigger back. onnce in battery, release the trigger and try dry fireing it again. if it fails to dry fire, I would inspect your holes you drilled for the trigger group and hammer very carfully. both sides of the rifle lower

check this link out. This might shed more light on it.



ar 15 trigger animation

I tried without the upper and works ok.  Then I put the upper back on and tried...it failed the dry fire.  Just out of curiosity I tried this procedure on the 223/556 frontier armory rifle and it failed the dry fire also!  What is interesting is that this very same rifle (223/556) worked fine cycling yesterday.  This lower (223/556) is the one I used to swap the 300 blkout onto....and worked/cycled fine.

only thing i can think of is its not going into full battery. if you feel the disconector disengage when you let the trigger go, the sear of the trigger is catching it.  next time try using the fwd assist after the 1st round to get it in full battery and see if it functions properly.