FFL/SOT here. Today I built up a BCI Defense receiver into an M16. I used a Bowers auto sear drill jig to make the hole. Its real easy for the jig to get crooked when tightening it down. Well I didn't realize it was not straight and had already drilled through 1/2 of the receiver so I kept going.

The sear pin hole is set slightly rearward of where it's supposed to be. Just to the right of the safety selector tick mark. Anyway, I finished building the lower and it seems to function fine when testing the trigger, safety, and sear.  However the sear just barely grabs the hammer.

I took it out to the range and the gun works fine on semi auto but turns into a single shot with hammer follow when the selector is on full auto. Tried a couple uppers with same issue. Using an H2 carbine buffer. Both uppers were new and worked fine on my other m16 lower.  All of the FCG springs look like they are in the correct places. Only thing I can think of would be the sear being slightly too far back. I'm just wondering why it seems to function fine when hand testing without an upper.

Is the lower finished?  if not Can't you fill in the holes with Tig weld and re machine? If it is finished Fastenal has something like JB Weld but a much smoother it sets up steel hard.
I used it on an 80% lower that I cut into the selector spring hole. I asked my weldor neighbor if he could weld it up then I could re machine but he said he could not get the tig head close enough.
I don't remember the name it was several years ago.

The timing is now jagged up, and if the auto sear is just touching the hammers auto sear hook...
When the bolt hits in battery, the shock can release it

I would plug it and weld over the plug, then get a KNS auto sear captive pin.

J.T. could help, or know a fix.

It is fixable, but you're a C3 so you would know that.

Thanks guys. I am pretty new to the manufacturer stuff. My second post sample that I've built and I dont have much in the way of welding or metal working equipment.

Andersons are $40.  Why not just start over?

Isn’t there a guy, “M60 Joe” on this site who does fixes like this?  John Thomas of Retro Arms Works may be another option.  I think it will have to be stripped of anodizing to weld the holes, then redrill.  Then, reanodizing or Ceracoat.  Or, plug with JB Weld and refinish with some spray finish.

The receiver you mention goes for about $100 - $150?

Yeah overall starting over isn't going to cost me a whole lot, just a hassle. Wanting to make sure something else couldn't be effecting the lower before I scrapped it.

drill out with undersized bit, throw a piece of 1/4" brass rod in there-re drill.

I'm not a SOT but I've timed a DIAS enough to know you need more than a little engagement or it wont work

I do not drill straight thru. I move the jig to the other side. YMMV.

**Edited** M60 Joe can probably help you out