Made by first M16 last night (FFL/SOT). Used a BCI Defense lower and a Bowers auto sear pin drill jig. Installed all new lower parts, an H2 buffer and new buffer spring.

Mated it to a Colt 14.5" upper... that is used with unknown history, but "looks" alright.

Took the gun to the range today for some function tests. First 3 20rnd magazines worked great an auto, no problems.  Part way through the 4th mag the hammer followed the bolt home resulting in a no fire situation. Ejected the round and things worked fine for the 5th mag.

Loaded up the six mag and the gun would run fine on semi auto but in auto mode with every round the hammer followed the bolt home and would not fire until recocked. Essentially giving me a single shot. Tried a couple more mags and same thing, every time.

Came home and manually function checked the lower. Everything seems to be working as is with the lower parts. So I'm thinking either an issue with the upper or something with the buffer and spring (but both were new).

Just seems really odd that it would work fine for a few mags and then stop. It was -5*F outside but I don't think that would effect much. The gun/ammo/mags were warm and only outside for less than 15 minutes. Any advice?

I've manufactured several form 10 guns for my department and have never had a problem. I've never used a jig before, I always mill the pocket and drill the cross hole. I would probably set the lower up in a mill and check the location to the factory Colt blueprints.

Sounds like the auto sear isn't engaging the hammer. That will result in hammer follow every time. Not thinking of anything in the upper that would cause it.

Incorrect timing dropping the hammer early would do it.

My money's on bolt (carrier) bounce, which can be exacerbated by incorrect timing.

First couple mags were breaking in the new components to the old, and after that things were running fast and regular enough to make the bolt bounce the dominant factor in the cycle.

Its possible, but a H2 is the correct buffer for an M4A1 barrel.

The auto sear engages the hammer just fine with the upper off the gun

Any chance it's short stroking while shooting? I would check the gas key.

Well could be a couple things, can you post pictures of the Bowers fixture?

The reality is only true way to drill sear hole is using a mill and measure off hammer hole.

It’s probably bolt bounce, can’t see it unless gun is firing... your hammer is barely being held by sear.

Remember different manufactures matchine hammer differently.... on the rear catch some are skinny and long and some are thick.... these all adjust timing.

I’ve had really good luck with Dpms m16 parts sets, if you need kne I might have a spare.

The fact is the hammer is following it forward, that is really bad.... you could take the sear out and get it to run full auto with right Ammo and upper, however it can and will have a out of battery explosion.

Post some pics, your hammer with sear engaged, jig and let’s see if we can get that bad boy running.

Oh yeah, buy the good heavy buffers for all your postys lowers.

Incorrect. OOB is a rare and odd bird in the AR world, owing to the telescoping design of the bolt and carrier. Simple hammer follow will not do it.

I would get a known to work upper, with quality ammo and try that first. I have never had a lower be the cause of the problems. It is always something in the upper.

Bolt Bounce in F/A is pretty rare, as the hammer is already coming home before the bolt bounces back much.

No that is not reality. Nobody I know of measures off the hammer hole. Never even heard of it till you brought it up. Some machinists without a jig will index off the selector hole, but none I know of use the hammer hole. The sear hole is actually a forgiving hole, a little here or there does not matter.

I and many others have drilled working sear holes with a DeWalt Drill and the Bowers fixture. Simple, inexpensive, works everytime.

That’s my suspicion. The used upper may have aggressive gas port erosion. If so, a h2 buffer may be insufficient. I had a well used Colt M4 upper that had massive port erosion. It was insanely overgassed to the point that a h2 did absolutely nothing. It had bolt bounce issues.

I ran into that same problem at work with old CQBR uppers from Crane. Two of them (so far) were still overgassed with H3 buffers. I'm sure we'll find more.

This is correct. I have made a bunch of M16's for my former employer. Every one located off the selector hole, every one ran like a champ.

Just wanted to give an update. I believe the problem was with the upper being over gassed. Lower works great after changing the configuration to an M16A1

Attached File

Just because you haven’t heard of it doesn’t make it reality. The truth is lowers can be out of spec, we all know this. By going off hammer hole you know everything will be perfect.

Sure you can use a jig, go off selector and etc. my first posty idk 20+ years ago we made a jig and used a hand drill.... gun worked perfect.

Not much time after I met with an old school C2 that we all know or have heard of, that is how they did it and had some really good insight why.

I’d still check your internal parts, no reason it should only work with a 20” upper.

On my factory M16, it started doing just like you describe when I installed a 11.5" upper and collapsible stock. It was bolt bounce. Ended up installing a Advanced Armament slow fire buffer that slowed the gun down and stopped the bounce.

It looks like the hammer is following the bolt, but in my case the hammer was falling when it should but the bolt was bounced enough out of battery to not get a good firing pin strike. If you take it back to the bench and everything is in spec (including timing), it's almost got to be bolt bounce.