Troubleshooting a new AR lower

[DenverGP]

Was troubleshooting a friend's 80% lower build. Was having "dead trigger" issues where the pulling the trigger allowed movement, but would not drop the hammer.

First thing I noticed was that the milling on the inside of the lower was not perfectly centered, so the trigger parts were slightly closer to the left side than the right. It caused some binding with the rear of the trigger where it fits with the notch in the safety. Turns out it doesn't bind when the safety is installed with the detent pin/spring since the safety slightly sticks out on the left side. So that apparently wasn't the issue.

I then realized that the disconnector appeared to just barely be engaging the hammer with the trigger released. Pushing down on the rear of the disconnector would release it, allowing the trigger pull to drop the hammer.

I swapped the hammer/trigger/disconnector with a spare I had onhand. The replacement hammer/trigger/disconnector functioned fine in the lower and seemed to resolve the problem.

Then I put the "bad" hammer/trigger/disconnector in my LaRue trigger action chassis, and they functioned perfectly fine there as well, but the gap (when trigger is not pulled) between the disconnector and hammer was almost non-existant, not sure if I could fit a sheet of paper between them.

My guess is the disconnector (or matching part of the hammer) was just barely out of spec, or the hammer/trigger pin hole positions on the 80% lower were just barely out spec. Or maybe even both in a case of tolerance stacking.

Anyone seen something like this? I think the problem is solved, and my friend is happy his gun is now functional. Trip to the range coming later this week to fully test.

[Great-Kazoo]

Who ever owns the lower, needs to take another pass, or 2 on the bottom of the lower. It's not uncommon to experience this issue. Also depends on who the H/T/S came from. Personally, i'd take a little more off the bottom.

[00tec]

Who ever owns the lower, needs to take another pass, or 2 on the bottom of the lower. It's not uncommon to experience this issue. Also depends on who the H/T/S came from. Personally, i'd take a little more off the bottom.

Yeppers.

The only issue I've ever had with an 80 is not milling out far enough. Centering or wall finish doesn't matter. If the floor isn't finished out, it will cause the tail of the trigger to hit the safety, even while it isn't set.

Take a caliper after it. You'll find that the floor is too thick, should be about the same as the wall

[DenverGP]

Appreciate the info. I'll take a caliper to it and measure the floor. As it sits now, it seems to be running fine, but I suspect it won't work with all triggers so if they swap parts it might have other issues.

[MarkCO]

My guess is the disconnector (or matching part of the hammer) was just barely out of spec, or the hammer/trigger pin hole positions on the 80% lower were just barely out spec. Or maybe even both in a case of tolerance stacking.

Anyone seen something like this? I think the problem is solved, and my friend is happy his gun is now functional. Trip to the range coming later this week to fully test.

I have run across several similar occurrences. Works in my lowers, not in their 80%. In every case, I was able to find something wrong in their lower. It was not much in some cases. Have one friend who just gave up, but most were able to fix what needed fixing. In one case, he had to drill out the trigger pin hole to oversized and install an insert to properly locate the hole.

[FromMyColdDeadHand]

Whose LPK?